if_epair: Also mark the flag of pair b with IFF_KNOWSEPOCH

Reviewed by:	kp
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D33210

sys/net/if_epair.c

@@ -629,6 +629,7 @@ epair_clone_create(struct if_clone *ifc, char *name, size_t len, caddr_t params)
 	ifp->if_dname = epairname;
 	ifp->if_dunit = unit;
 	ifp->if_flags = IFF_BROADCAST | IFF_SIMPLEX | IFF_MULTICAST;
+	ifp->if_flags |= IFF_KNOWSEPOCH;
 	ifp->if_capabilities = IFCAP_VLAN_MTU;
 	ifp->if_capenable = IFCAP_VLAN_MTU;
 	ifp->if_start = epair_start;