Instead of creating a purge thread for every vnet, create
a single purge thread and clean up all vnets from this thread.

(1) Boot a kernel with VIMAGE enabled
(2) Create a vnet jail

jail -c persist name=testjail001 vnet path=/ host.hostname=testjail001 allow.raw_sockets allow.socket_af

(3) Start pf inside the jail

service start pf

(4) Delete the vnet jail

jail -r testjail001

Without this patch, the kernel would panic in step (4). With the patch,
the kernel does not panic