f2fs: avoid to trigger writepage during POR

This patch doesn't make any effect on previous behavior, since
f2fs_write_data_page bypasses writing the page during POR.

But, the difference is that this patch avoids holding writepages mutex.
This is to avoid the following false warning, since this can happen only
when mount and shutdown are triggered at the same time.

 ======================================================
 [ INFO: possible circular locking dependency detected ]
 4.0.0-rc1+ #3 Tainted: G           O
 -------------------------------------------------------
 kworker/u8:0/2270 is trying to acquire lock:
  (&sbi->gc_mutex){+.+.+.}, at: [<ffffffffa02bdd33>] f2fs_balance_fs+0x73/0x90 [f2fs]

 but task is already holding lock:
  (&sbi->writepages){+.+...}, at: [<ffffffffa02b261b>] f2fs_write_data_pages+0xcb/0x3a0 [f2fs]

 which lock already depends on the new lock.

 the existing dependency chain (in reverse order) is:

 -> #2 (&sbi->writepages){+.+...}:
        [<ffffffff810e2b11>] lock_acquire+0xe1/0x2f0
        [<ffffffff8185e1b3>] mutex_lock_nested+0x63/0x530
        [<ffffffffa02b261b>] f2fs_write_data_pages+0xcb/0x3a0 [f2fs]
        [<ffffffff811c38c1>] do_writepages+0x21/0x50
        [<ffffffff8126c5a6>] __writeback_single_inode+0x76/0xbf0
        [<ffffffff8126e23a>] writeback_single_inode+0xea/0x1c0
        [<ffffffff8126e425>] write_inode_now+0x95/0xa0
        [<ffffffff81259dab>] iput+0x20b/0x3f0
        [<ffffffffa02c1c8b>] recover_data.constprop.14+0x26b/0xa80 [f2fs]
        [<ffffffffa02c2776>] recover_fsync_data+0x2b6/0x5e0 [f2fs]
        [<ffffffffa02a9744>] f2fs_fill_super+0xb24/0xb90 [f2fs]
        [<ffffffff8123d7f4>] mount_bdev+0x1a4/0x1e0
        [<ffffffffa02a3c85>] f2fs_mount+0x15/0x20 [f2fs]
        [<ffffffff8123e159>] mount_fs+0x39/0x180
        [<ffffffff8125e51b>] vfs_kern_mount+0x6b/0x160
        [<ffffffff81261554>] do_mount+0x204/0xbe0
        [<ffffffff8126223b>] SyS_mount+0x8b/0xe0
        [<ffffffff81863e6d>] system_call_fastpath+0x16/0x1b

 -> #1 (&sbi->cp_mutex){+.+...}:
        [<ffffffff810e2b11>] lock_acquire+0xe1/0x2f0
        [<ffffffff8185e1b3>] mutex_lock_nested+0x63/0x530
        [<ffffffffa02acbf2>] write_checkpoint+0x42/0x1230 [f2fs]
        [<ffffffffa02a847d>] f2fs_sync_fs+0x9d/0x2a0 [f2fs]
        [<ffffffff81272f82>] sync_filesystem+0x82/0xb0
        [<ffffffff8123c214>] generic_shutdown_super+0x34/0x100
        [<ffffffff8123c5f7>] kill_block_super+0x27/0x70
        [<ffffffffa02a3c60>] kill_f2fs_super+0x20/0x30 [f2fs]
        [<ffffffff8123ca49>] deactivate_locked_super+0x49/0x80
        [<ffffffff8123d05e>] deactivate_super+0x4e/0x70
        [<ffffffff8125df63>] cleanup_mnt+0x43/0x90
        [<ffffffff8125e002>] __cleanup_mnt+0x12/0x20
        [<ffffffff810a82e4>] task_work_run+0xc4/0xf0
        [<ffffffff8101f0bd>] do_notify_resume+0x8d/0xa0
        [<ffffffff81864141>] int_signal+0x12/0x17

 -> #0 (&sbi->gc_mutex){+.+.+.}:
        [<ffffffff810e2866>] __lock_acquire+0x1ac6/0x1c90
        [<ffffffff810e2b11>] lock_acquire+0xe1/0x2f0
        [<ffffffff8185e1b3>] mutex_lock_nested+0x63/0x530
        [<ffffffffa02bdd33>] f2fs_balance_fs+0x73/0x90 [f2fs]
        [<ffffffffa02b5938>] f2fs_write_data_page+0x348/0x5b0 [f2fs]
        [<ffffffffa02af9da>] __f2fs_writepage+0x1a/0x50 [f2fs]
        [<ffffffff811c1b54>] write_cache_pages+0x274/0x6f0
        [<ffffffffa02b2630>] f2fs_write_data_pages+0xe0/0x3a0 [f2fs]
        [<ffffffff811c38c1>] do_writepages+0x21/0x50
        [<ffffffff8126c5a6>] __writeback_single_inode+0x76/0xbf0
        [<ffffffff8126d44a>] writeback_sb_inodes+0x32a/0x710
        [<ffffffff8126d8cf>] __writeback_inodes_wb+0x9f/0xd0
        [<ffffffff8126dcdb>] wb_writeback+0x3db/0x850
        [<ffffffff8126e848>] bdi_writeback_workfn+0x148/0x980
        [<ffffffff810a3782>] process_one_work+0x1e2/0x840
        [<ffffffff810a3f01>] worker_thread+0x121/0x460
        [<ffffffff810a9dc8>] kthread+0xf8/0x110
        [<ffffffff81863dbc>] ret_from_fork+0x7c/0xb0

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>