f2fs: declare nested quota_sem and remove unnecessary sems

1.
f2fs_quota_sync
 -> down_read(&sbi->quota_sem)
 -> dquot_writeback_dquots
  -> f2fs_dquot_commit
   -> down_read(&sbi->quota_sem)

2.
f2fs_quota_sync
 -> down_read(&sbi->quota_sem)
  -> f2fs_write_data_pages
   -> f2fs_write_single_data_page
    -> down_write(&F2FS_I(inode)->i_sem)

f2fs_mkdir
 -> f2fs_do_add_link
   -> down_write(&F2FS_I(inode)->i_sem)
   -> f2fs_init_inode_metadata
    -> f2fs_new_node_page
     -> dquot_alloc_inode
      -> f2fs_dquot_mark_dquot_dirty
       -> down_read(&sbi->quota_sem)

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