In userland, sign extend the offset for JA instructions.

We currently use that to implement "ip6 protochain", and "pc" might be
wider than "pc->k", in which case we need to arrange that "pc->k" be
sign-extended, by casting it to bpf_int32.

bpf/net/bpf_filter.c

@@ -405,7 +405,18 @@ bpf_filter(pc, p, wirelen, buflen)
 			continue;
 
 		case BPF_JMP|BPF_JA:
+#if defined(KERNEL) || defined(_KERNEL)
+			/*
+			 * No backward jumps allowed.
+			 */
 			pc += pc->k;
+#else
+			/*
+			 * XXX - we currently implement "ip6 protochain"
+			 * with backward jumps, so sign-extend pc->k.
+			 */
+			pc += (bpf_int32)pc->k;
+#endif
 			continue;
 
 		case BPF_JMP|BPF_JGT|BPF_K: