f2fs: call unlock_new_inode() before d_instantiate()

xfstest generic/429 sometimes hangs on f2fs, caused by a thread being
unable to take a directory's i_rwsem for write in vfs_rmdir().  In the
test, one thread repeatedly creates and removes a directory, and other
threads repeatedly look up a file in the directory.  The bug is that
f2fs_mkdir() calls d_instantiate() before unlock_new_inode(), resulting
in the directory inode being exposed to lookups before it has been fully
initialized.  And with CONFIG_DEBUG_LOCK_ALLOC, unlock_new_inode()
reinitializes ->i_rwsem, corrupting its state when it is already held.

Fix it by calling unlock_new_inode() before d_instantiate().  This
matches what other filesystems do.

Fixes: 57397d86 ("f2fs: add inode operations for special inodes")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>