MFC r258588

MFC r258588

In sys/netpfil/ipfw/ip_fw_nat.c:vnet_ipfw_nat_uninit() we call "IPFW_WLOCK(chain);".
This lock gets deleted in sys/netpfil/ipfw/ip_fw2.c:vnet_ipfw_uninit().

Therefore, vnet_ipfw_nat_uninit() *must* be called before vnet_ipfw_uninit(),
but this doesn't always happen, because the VNET_SYSINIT order is the same for both functions.
In sys/net/netpfil/ipfw/ip_fw2.c and sys/net/netpfil/ipfw/ip_fw_nat.c,
IPFW_SI_SUB_FIREWALL == IPFW_NAT_SI_SUB_FIREWALL == SI_SUB_PROTO_IFATTACHDOMAIN
and
IPFW_MODULE_ORDER == IPFW_NAT_MODULE_ORDER

Consequently, if VIMAGE is enabled, and jails are created and destroyed,
the system sometimes crashes, because we are trying to use a deleted lock.

To reproduce the problem:
  (1)  Take a GENERIC kernel config, and add options for: VIMAGE, WITNESS,
       INVARIANTS.
  (2)  Run this command in a loop:
       jail -l -u root -c path=/ name=foo persist vnet && jexec foo ifconfig lo0 127.0.0.1/8 && jail -r foo

       (see http://lists.freebsd.org/pipermail/freebsd-current/2010-November/021280.html )

Fix the problem by increasing the value of IPFW_NAT_SI_SUB_FIREWALL,
so that vnet_ipfw_nat_uninit() runs after vnet_ipfw_uninit().

Approved by: re (gjb)

git-svn-id: svn://svn.freebsd.org/base/stable/10@258912 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f