Assert the state of the process lock and sigact mutex in
kern_sigprocmask() and reschedule_signals().

Discussed with:	rea
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275206 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f