init: stricter checking of major:minor root= values

In the kernel command-line, previously, root=1:2jakshflaksjdhfa would
be accepted and interpreted just like root=1:2. This patch adds
stricter checking so that additional characters after major:minor are
rejected by root=.

The goal of this change is to help in unifying DM's interpretation of
its block device argument by using existing kernel code (name_to_dev_t).
But DM rejects malformed major:minor pairs, it seems reasonable for
root= to reject them as well.
Signed-off-by: NDan Ehrenberg <dehrenberg@chromium.org>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

init/do_mounts.c

@@ -226,8 +226,9 @@ dev_t name_to_dev_t(const char *name)
 
 	if (strncmp(name, "/dev/", 5) != 0) {
 		unsigned maj, min;
+		char dummy;
 
-		if (sscanf(name, "%u:%u", &maj, &min) == 2) {
+		if (sscanf(name, "%u:%u%c", &maj, &min, &dummy) == 2) {
 			res = MKDEV(maj, min);
 			if (maj != MAJOR(res) || min != MINOR(res))
 				goto fail;