fs/super.c: improve get_tree() error message

seeing an odd bug where we fail to correctly return an error from
.get_tree():

https://syzkaller.appspot.com/bug?extid=c0360e8367d6d8d04a66

we need to be able to distinguish between accidently returning a
positive error (as implied by the log) and no error.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/super.c

@@ -1802,8 +1802,8 @@ int vfs_get_tree(struct fs_context *fc)
 		return error;
 
 	if (!fc->root) {
-		pr_err("Filesystem %s get_tree() didn't set fc->root\n",
-		       fc->fs_type->name);
+		pr_err("Filesystem %s get_tree() didn't set fc->root, returned %i\n",
+		       fc->fs_type->name, error);
 		/* We don't know what the locking state of the superblock is -
 		 * if there is a superblock.
 		 */