AIX: Use afs_IsPagId()

Commit c856bd9c87 (afs: Introduce afs_IsPagId()) converted all of our
PAG id checks to use afs_IsPagId(), except for one place in AIX's
osi_groups.c by mistake. Update it, so all PAG id checks now use
afs_IsPagId().

Thanks to ben@huntsmans.net for pointing out the mistake.

Change-Id: Ic18967da229d60b7c9fab8da565dd80645ac1c7f
Reviewed-on: https://gerrit.openafs.org/16116
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Ben Huntsman <ben@huntsmans.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>

src/afs/AIX/osi_groups.c

@@ -60,7 +60,7 @@ setgroups(ngroups, gidset)
     credp0 = credp;
 
     if (PagInCred(credp) == NOPAG) {
-	if (((treq.uid >> 24) & 0xff) == 'A') {
+	if (afs_IsPagId(treq.uid)) {
 	    AFS_GLOCK();
 	    AddPag(treq.uid, &credp);
 	    AFS_GUNLOCK();