depmod: Don't fall back to uname on bad version

Currently, if a value that doesn't match a kernel version
("%u.%u") is passed in, depmod silently falls back to
using uname. Rather than try and work around the caller passing
bad data, just exit out instead.
diff --git a/tools/depmod.c b/tools/depmod.c
index 348735f..a585d47 100644
--- a/tools/depmod.c
+++ b/tools/depmod.c
@@ -2476,7 +2476,11 @@
 		}
 	}
 
-	if (optind < argc && is_version_number(argv[optind])) {
+	if (optind < argc) {
+		if (!is_version_number(argv[optind])) {
+			ERR("Bad version passed %s\n", argv[optind]);
+			goto cmdline_failed;
+		}
 		cfg.kversion = argv[optind];
 		optind++;
 	} else {