In udp_input() when walking the pcblist we can come across

an inp marked FREED after the epoch(9) changes.
Check once we hold the lock and skip the inp if it is the case.

Contrary to IPv6 the locking of the inp is outside the multicast
section and hence a single check seems to suffice.

PR:		232192
Reviewed by:	mmacy, markj
Approved by:	re (kib)
Differential Revision:	https://reviews.freebsd.org/D17540

sys/netinet/udp_usrreq.c

@@ -551,6 +551,11 @@ udp_input(struct mbuf **mp, int *offp, int proto)
 
 			INP_RLOCK(inp);
 
+			if (__predict_false(inp->inp_flags2 & INP_FREED)) {
+				INP_RUNLOCK(inp);
+				continue;
+			}
+
 			/*
 			 * XXXRW: Because we weren't holding either the inpcb
 			 * or the hash lock when we checked for a match