f2fs: fix extent corrupotion during directIO in LFS mode

In LFS mode, por_fsstress testcase reports a bug as below:

[ASSERT] (fsck_chk_inode_blk: 931)  --> ino: 0x12fe has wrong ext: [pgofs:142, blk:215424, len:16]

Since commit f847c699 ("f2fs: allow out-place-update for direct
IO in LFS mode"), we start to allow OPU mode for direct IO, however,
we missed to update extent cache in __allocate_data_block(), finally,
it cause extent field being inconsistent with physical block address,
fix it.

Fixes: f847c699 ("f2fs: allow out-place-update for direct IO in LFS mode")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>