autofs: add WARN_ON(1) for non dir/link inode case

It's invalid if the given mode is neither dir nor link, so warn on else
case.

Link: http://lkml.kernel.org/r/20160812024754.12352.8536.stgit@pluto.themaw.net
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/autofs4/inode.c

@@ -368,7 +368,8 @@ struct inode *autofs4_get_inode(struct super_block *sb, umode_t mode)
 		inode->i_fop = &autofs4_dir_operations;
 	} else if (S_ISLNK(mode)) {
 		inode->i_op = &autofs4_symlink_inode_operations;
-	}
+	} else
+		WARN_ON(1);
 
 	return inode;
 }