vol: Correct VolumeNumber for large volume IDs

VolumeNumber was using atoi to convert a volume header name to a
volume ID. This can return just -1 for volume IDs larger than 2^31-1,
though, so use strtoul instead.

Reviewed-on: http://gerrit.openafs.org/4199
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 0f1c921fa1)

Change-Id: Ib56d3f77f0d90f5ad55b3c0de89632d271436f7e
Reviewed-on: http://gerrit.openafs.org/4284
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>

src/vol/volume.c

@@ -6058,7 +6058,7 @@ VolumeNumber(char *name)
 {
     if (*name == OS_DIRSEPC)
 	name++;
-    return atoi(name + 1);
+    return strtoul(name + 1, NULL, 10);
 }
 
 /**