acquire inp lock around ip6_pcbopt to fix IPV6_TCLASS panic

Simple fix to address panics relating to setting IPV6_TCLASS
with setsockopt(). The premise of this change is that it is
ok to call malloc with M_NOWAIT while holding a lock on the
in6p.

If it later turns out that it is not ok, then major surgery
will be required, as ip6_setpktopt() will have to be fixed
(as it also calls malloc with M_NOWAIT) which pulls in the
ip6_pcbopts(), ip6_setpktopts(), ip6_setpktopt() call chain.

Submitted by:	Jason Eggnet
Reviewed by:	rrs, transport, sbruno
Sponsored by:	Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D16201

sys/netinet6/ip6_output.c

@@ -1636,11 +1636,17 @@ do {									\
 						error = EINVAL;
 						break;
 					}
+					INP_WLOCK(in6p);
+					if (in6p->inp_flags & (INP_TIMEWAIT | INP_DROPPED)) {
+						INP_WUNLOCK(in6p);
+						return (ECONNRESET);
+					}
 					optp = &in6p->in6p_outputopts;
 					error = ip6_pcbopt(IPV6_HOPLIMIT,
 					    (u_char *)&optval, sizeof(optval),
 					    optp, (td != NULL) ? td->td_ucred :
 					    NULL, uproto);
+					INP_WUNLOCK(in6p);
 					break;
 				}
 
@@ -1750,11 +1756,17 @@ do {									\
 					break;
 				{
 					struct ip6_pktopts **optp;
+					INP_WLOCK(in6p);
+					if (in6p->inp_flags & (INP_TIMEWAIT | INP_DROPPED)) {
+						INP_WUNLOCK(in6p);
+						return (ECONNRESET);
+					}
 					optp = &in6p->in6p_outputopts;
 					error = ip6_pcbopt(optname,
 					    (u_char *)&optval, sizeof(optval),
 					    optp, (td != NULL) ? td->td_ucred :
 					    NULL, uproto);
+					INP_WUNLOCK(in6p);
 					break;
 				}
 
@@ -1836,10 +1848,16 @@ do {									\
 					break;
 				optlen = sopt->sopt_valsize;
 				optbuf = optbuf_storage;
+				INP_WLOCK(in6p);
+				if (in6p->inp_flags & (INP_TIMEWAIT | INP_DROPPED)) {
+					INP_WUNLOCK(in6p);
+					return (ECONNRESET);
+				}
 				optp = &in6p->in6p_outputopts;
 				error = ip6_pcbopt(optname, optbuf, optlen,
 				    optp, (td != NULL) ? td->td_ucred : NULL,
 				    uproto);
+				INP_WUNLOCK(in6p);
 				break;
 			}
 #undef OPTSET
@@ -2286,7 +2304,9 @@ ip6_pcbopt(int optname, u_char *buf, int len, struct ip6_pktopts **pktopt,
 
 	if (*pktopt == NULL) {
 		*pktopt = malloc(sizeof(struct ip6_pktopts), M_IP6OPT,
-		    M_WAITOK);
+		    M_NOWAIT);
+		if (*pktopt == NULL)
+			return (ENOBUFS);
 		ip6_initpktopts(*pktopt);
 	}
 	opt = *pktopt;