ipfilter: Avoid holding a lock while stopping

Avoid calling _callout_stop_safe with a non-sleepable lock held when
detaching by initializing callout_init_rw() with CALLOUT_SHAREDLOCK, and
avoiding re-initialization inside the timer function.

PR: 282478
Reviewed by: cy, emaste, jhb, markj
Tested by: cy
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47530