Fix duplicate acquiring of refcount when joining IPv6 multicast groups.

This was observed by starting and stopping rpcbind(8) multiple times.

PR:			233535
Differential Revision:	https://reviews.freebsd.org/D18887
Reviewed by:		bz (net)
Tested by:		ae
MFC after:		1 week
Sponsored by:		Mellanox Technologies

sys/netinet6/in6_mcast.c

@@ -2178,7 +2178,10 @@ in6p_join_group(struct inpcb *inp, struct sockopt *sopt)
 			IN6_MULTI_UNLOCK();
 			goto out_im6o_free;
 		}
-		in6m_acquire(inm);
+		/*
+		 * NOTE: Refcount from in6_joingroup_locked()
+		 * is protecting membership.
+		 */
 		imo->im6o_membership[idx] = inm;
 	} else {
 		CTR1(KTR_MLD, "%s: merge inm state", __func__);