net: bandaid for plugging a fw_com leak in fwip_detach()

Adding a temporary workaround for plugging a fw_com upon if_fwip unloading.

Steps to reproduce(needs two hosts connected with firewire):

  while true; do
    ifconfig fwip0 10.0.0.5 up
    fwcontrol -r
    ping -c 10.0.0.3
    kldunload if_fwip
  done

There's a chance that the unloading of if_fwip.ko triggers following warning:

	Warning: memory type fw_com leaked memory on destroy (1 allocations, 64 bytes leaked).

commit d79b6b8ec2 (origin/main, origin/HEAD)

(cherry picked from commit 25a5bb7318)

sys/net/if_fwsubr.c

@@ -801,6 +801,7 @@ firewire_ifdetach(struct ifnet *ifp)
 {
 	bpfdetach(ifp);
 	if_detach(ifp);
+	NET_EPOCH_DRAIN_CALLBACKS();
 }
 
 void