tcp rack, bbr: improve handling of soft errors

Do not report an error, if it is stored as a soft error. This avoids,
for example, the dropping of TCP connections using an interface,
while enabling or disabling LRO on that interface.

Reviewed by:		cc
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D46427

(cherry picked from commit b2044c4557443bbce974101f04e2b465d1bbe769)

sys/netinet/tcp_stacks/bbr.c

@@ -13837,6 +13837,7 @@ nomore:
 		case ENETUNREACH:
 			if (TCPS_HAVERCVDSYN(tp->t_state)) {
 				tp->t_softerror = error;
+				error = 0;
 			}
 			/* FALLTHROUGH */
 		default:

sys/netinet/tcp_stacks/rack.c

@@ -22473,6 +22473,7 @@ nomore:
 		case ENETDOWN:
 			if (TCPS_HAVERCVDSYN(tp->t_state)) {
 				tp->t_softerror = error;
+				error = 0;
 			}
 			/* FALLTHROUGH */
 		default: