f2fs: fix i_current_depth during inline dentry conversion

With below steps, we will see that dentry page becoming unaccessable later.
This is because we forget updating i_current_depth in inode during inline
dentry conversion, after that, once we failed at somewhere, it will leave
i_current_depth as 0 in non-inline directory. Then, during ->lookup, the
current_depth value makes all dentry pages in first level invisible. Fix
it.

1) mount f2fs with inline_dentry option
2) mkdir dir
3) touch 180 files named [0-179] in dir
4) touch 180 in dir (fail after inline dir conversion)
5) ll dir

ls: cannot access /mnt/f2fs/dir/0: No such file or directory
ls: cannot access /mnt/f2fs/dir/1: No such file or directory
ls: cannot access /mnt/f2fs/dir/2: No such file or directory
ls: cannot access /mnt/f2fs/dir/3: No such file or directory
ls: cannot access /mnt/f2fs/dir/4: No such file or directory

drwxr-xr-x 2 root root 4096  may 13 21:47 ./
drwxr-xr-x 3 root root 4096  may 13 21:46 ../
-????????? ? ?    ?       ?             ? 0
-????????? ? ?    ?       ?             ? 1
-????????? ? ?    ?       ?             ? 10
-????????? ? ?    ?       ?             ? 100
-????????? ? ?    ?       ?             ? 101
-????????? ? ?    ?       ?             ? 102

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