MFC r292254:

MFC r292254:

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.

PR:			208171
Requested by:		Franco Fichtner (opnsense.org)
Differential Revision:	https://reviews.freebsd.org/D3855

git-svn-id: svn://svn.freebsd.org/base/stable/10@297228 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f