in6_mcast: Acquire ref to ifp and exit epoch earlier in sysctl

This patch reduces the number of witness warnings during ifmcstat(8) calls.

Reviewed by: glebius, zlei
Differential Revision: https://reviews.freebsd.org/D56052

sys/netinet6/in6_mcast.c

@@ -2806,9 +2806,9 @@ sysctl_ip6_mcast_filters(SYSCTL_HANDLER_ARGS)
 
 	ifindex = name[0];
 	NET_EPOCH_ENTER(et);
-	ifp = ifnet_byindex(ifindex);
+	ifp = ifnet_byindex_ref(ifindex);
+	NET_EPOCH_EXIT(et);
 	if (ifp == NULL) {
-		NET_EPOCH_EXIT(et);
 		CTR2(KTR_MLD, "%s: no ifp for ifindex %u",
 		    __func__, ifindex);
 		return (ENOENT);
@@ -2821,7 +2821,7 @@ sysctl_ip6_mcast_filters(SYSCTL_HANDLER_ARGS)
 	retval = sysctl_wire_old_buffer(req,
 	    sizeof(uint32_t) + (in6_mcast_maxgrpsrc * sizeof(struct in6_addr)));
 	if (retval) {
-		NET_EPOCH_EXIT(et);
+		if_rele(ifp);
 		return (retval);
 	}
 
@@ -2856,7 +2856,7 @@ sysctl_ip6_mcast_filters(SYSCTL_HANDLER_ARGS)
 	}
 	IN6_MULTI_LIST_UNLOCK();
 	IN6_MULTI_UNLOCK();
-	NET_EPOCH_EXIT(et);
+	if_rele(ifp);
 
 	return (retval);
 }