depmod: Do not accept relative moduledir

This makes for a confusing interface, particularly considering that -b
can also be set, with or without a trailing slash.

Since the build system already enforces an absolute path, do the same when
passing it via command line, avoiding the following error:

	$ sudo ln -s /lib/modules /foobar
	$ depmod -m foobar -o /tmp/result2 6.11.7
	depmod: ERROR: could not open directory foobar/6.11.7: No such file or directory
	depmod: FATAL: could not search modules: No such file or directory

Signed-off-by: Lucas De Marchi <[email protected]>

tools/depmod.c

@@ -2961,6 +2961,11 @@ static int do_depmod(int argc, char *argv[])
 			}
 			break;
 		case 'm':
+			if (optarg[0] != '/') {
+				ERR("invalid module directory '%s': absolute path required",
+				    optarg);
+				goto cmdline_failed;
+			}
 			module_directory = optarg;
 			break;
 		case 'o':