fs: no need to check source

The @source inode must be valid. It is even checked via IS_SWAPFILE()
above making it pretty clear. So no need to check it when we unlock.

What doesn't need to exist is the @target inode. The lock_two_inodes()
helper currently swaps the @inode1 and @inode2 arguments if @inode1 is
NULL to have consistent lock class usage. However, we know that at least
for vfs_rename() that @inode1 is @source and thus is never NULL as per
above. We also know that @source is a different inode than @target as
that is checked right at the beginning of vfs_rename(). So we know that
@source is valid and locked and that @target is locked. So drop the
check whether @source is non-NULL.

Fixes: 28eceeda130f ("fs: Lock moved directories")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202307030026.9sE2pk2x-lkp@intel.com
Message-Id: <20230703-vfs-rename-source-v1-1-37eebb29b65b@kernel.org>
[brauner: use commit message from patch I sent concurrently]
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namei.c

@@ -4874,8 +4874,7 @@ int vfs_rename(struct renamedata *rd)
 			d_exchange(old_dentry, new_dentry);
 	}
 out:
-	if (source)
-		inode_unlock(source);
+	inode_unlock(source);
 	if (target)
 		inode_unlock(target);
 	dput(new_dentry);