Properly drain callouts in the IPFW subsystem to avoid use after free
panics when unloading the dummynet and IPFW modules:

• The callout drain function can sleep and should not be called having

a non-sleepable lock locked. Remove locks around "ipfw_dyn_uninit(0)".

• Add a new "dn_gone" variable to prevent asynchronous restart of

dummynet callouts when unloading the dummynet kernel module.

• Call "dn_reschedule()" locked so that "dn_gone" can be set and

checked atomically with regard to starting a new callout.

Reviewed by: hiren
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D3855