lockmgr: upgrade panic return checks

We short-circuit lockmgr functions in the face of a kernel panic. Other
lock implementations do this with a SCHEDULER_STOPPED() check, which
covers the additional case where the debugger is active but the system
has not panicked. Update this code to match that behaviour.

Reviewed by: mjg, kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38655

(cherry picked from commit 9a7f7c26c5b280924072e2d9947ad39ef4cc022c)