libprocstat: procstat_getgroups_kvm(): Output again the effective GID

In particular, fixes 'procstat -s -M' (for processes with more than 16
groups).

Reviewed by:    kib
Fixes:          be1f7435ef ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after:      5 days
MFC to:         stable/15
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D52275

lib/libprocstat/libprocstat.c

@@ -1973,6 +1973,7 @@ procstat_getgroups_kvm(kvm_t *kd, struct kinfo_proc *kp, unsigned int *cntp)
 	struct ucred ucred;
 	gid_t *groups;
 	size_t len;
+	unsigned int ngroups;
 
 	assert(kd != NULL);
 	assert(kp != NULL);
@@ -1990,19 +1991,22 @@ procstat_getgroups_kvm(kvm_t *kd, struct kinfo_proc *kp, unsigned int *cntp)
 		    proc.p_ucred, kp->ki_pid);
 		return (NULL);
 	}
-	len = ucred.cr_ngroups * sizeof(gid_t);
+	ngroups = 1 + ucred.cr_ngroups;
+	len = ngroups * sizeof(gid_t);
 	groups = malloc(len);
 	if (groups == NULL) {
 		warn("malloc(%zu)", len);
 		return (NULL);
 	}
-	if (!kvm_read_all(kd, (unsigned long)ucred.cr_groups, groups, len)) {
+	groups[0] = ucred.cr_gid;
+	if (!kvm_read_all(kd, (unsigned long)ucred.cr_groups, groups + 1,
+	    len - sizeof(gid_t))) {
 		warnx("can't read groups at %p for pid %d",
 		    ucred.cr_groups, kp->ki_pid);
 		free(groups);
 		return (NULL);
 	}
-	*cntp = ucred.cr_ngroups;
+	*cntp = ngroups;
 	return (groups);
 }