loopif: set pbuf if_idx during poll

This sets the pbuf's if_idx during the loopif poll function (the
equivalent netif input function).  This was found during IP_PKTINFO
development where p->if_idx is read and was uninitialized

src/core/netif.c

@@ -1057,6 +1057,8 @@ netif_poll(struct netif *netif)
     in_end->next = NULL;
     SYS_ARCH_UNPROTECT(lev);
 
+    in->if_idx = netif_get_index(netif);
+
     LINK_STATS_INC(link.recv);
     MIB2_STATS_NETIF_ADD(stats_if, ifinoctets, in->tot_len);
     MIB2_STATS_NETIF_INC(stats_if, ifinucastpkts);