lagg: Avoid adding a port to a lagg device being destroyed.

The lagg_clone_destroy() handles detach and waiting for ifconfig callers
to drain already.

This narrows the race for 2 panics that the tests triggered. Both were a
consequence of adding a port to the lagg device after it had already detached
from all of its ports. The link state task would run after lagg_clone_destroy()
free'd the lagg softc.

    kernel:trap_fatal+0xa4
    kernel:trap_pfault+0x61
    kernel:trap+0x316
    kernel:witness_checkorder+0x6d
    kernel:_sx_xlock+0x72
    if_lagg.ko:lagg_port_state+0x3b
    kernel:if_down+0x144
    kernel:if_detach+0x659
    if_tap.ko:tap_destroy+0x46
    kernel:if_clone_destroyif+0x1b7
    kernel:if_clone_destroy+0x8d
    kernel:ifioctl+0x29c
    kernel:kern_ioctl+0x2bd
    kernel:sys_ioctl+0x16d
    kernel:amd64_syscall+0x337

    kernel:trap_fatal+0xa4
    kernel:trap_pfault+0x61
    kernel:trap+0x316
    kernel:witness_checkorder+0x6d
    kernel:_sx_xlock+0x72
    if_lagg.ko:lagg_port_state+0x3b
    kernel:do_link_state_change+0x9b
    kernel:taskqueue_run_locked+0x10b
    kernel:taskqueue_run+0x49
    kernel:ithread_loop+0x19c
    kernel:fork_exit+0x83

PR:		244168
Reviewed by:	markj
MFC after:	2 weeks
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D25284

sys/net/if_lagg.c

@@ -679,6 +679,9 @@ lagg_port_create(struct lagg_softc *sc, struct ifnet *ifp)
 		return (EINVAL);
 	}
 
+	if (sc->sc_destroying == 1)
+		return (ENXIO);
+
 	/* Limit the maximal number of lagg ports */
 	if (sc->sc_count >= LAGG_MAX_PORTS)
 		return (ENOSPC);
@@ -1191,6 +1194,8 @@ lagg_ioctl(struct ifnet *ifp, u_long cmd, caddr_t data)
 
 	bzero(&rpbuf, sizeof(rpbuf));
 
+	/* XXX: This can race with lagg_clone_destroy. */
+
 	switch (cmd) {
 	case SIOCGLAGG:
 		LAGG_XLOCK(sc);