f2fs: avoid writing inode redundantly when creating a file

In f2fs_write_inode, updating inode after f2fs_balance_fs is not
a optimized way in the case that f2fs_gc is performed ahead. The
inode page will be unnecessarily written out twice, one of which
is in f2fs_gc->...->sync_node_pages and the other is in
update_inode_page.

Let's update the inode page in prior to f2fs_balance_fs to avoid
this.

To reproduce it,
$ touch file (before this step, should make the device need f2fs_gc)
$ sync (or wait the bdi to write dirty inode)

Signed-off-by: Jin Xu <jinuxstyle@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>