f2fs: fix readdir incorrectness

In the previous Al Viro's readdir patch set, there occurs a bug when
running
xfstest: 006 as follows.

[Error output]
alpha size = 4, name length = 6, total files = 4096, nproc=1
1023 files created
rm: cannot remove `/mnt/f2fs/permname.15150/a': Directory not empty

[Correct output]
alpha size = 4, name length = 6, total files = 4096, nproc=1
4097 files created

This bug is due to the misupdate of directory position in ctx.
So, this patch fixes this.

[AV: fixed a braino]

CC: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>