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