f2fs: update i_size after DIO completion

This is related to
ee70daab ("xfs: update i_size after unwritten conversion in dio completion")

If we update i_size during dio_write, dio_read can read out stale data, which
breaks xfstests/465.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>