Always stop the scheduler when entering kdb

Set curthread->td_stopsched when entering kdb via any vector.
Previously, it was only set when entering via panic, so when
entering kdb another way, mutexes and such were still "live",
and an attempt to lock an already locked mutex would panic.

Reviewed by: kib, cem
Discussed with: jhb
Tested by: pho
MFC after: 2 months
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17687