pf: always mark states as unlinked before detaching them

Users have reported crashes in pf_test_state_udp() where at least one state key
is NULL.

That suggests that pf_detach_state() ran concurrently with pf_test_state_udp().
pf_test_state_udp() holds the state lock (aka the id lock), but
pf_detach_state() does not.
The intent is that detached states are not returned by STATE_LOOKUP/
pf_find_state(), as the state's timeout is set to PFTM_UNLINKED and thus
pf_find_state() does not find the state.

There are other paths to pf_detach_state() (outside of pf_unlink_state())
though, where we did not set the timeout to PFTM_UNLINKED. Fix those, and assert
that the timeout is set correctly when we enter pf_detach_state().

MFC after: 1 week
See also: https://redmine.pfsense.org/issues/15413
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D45101