check_cancel: when in_sigsuspend, send SIGCANCEL unconditionally

not only when check_cancel is called from the signal handler.
_thr_wake() is not enough to get the sigsuspend(2) syscall out of the
loop, since it only terminates sleep when an unblocked signal is
delivered.

PR: 283101
In collaboration with: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D48200