sleepq_catch_signals: do thread suspension before signal check

sleepq_catch_signals: do thread suspension before signal check

Since locks are dropped when a thread suspends, it's possible for another
thread to deliver a signal to the suspended thread. If the thread awakens from
suspension without checking for signals, it may go to sleep despite having
a pending signal that should wake it up. Therefore the suspension check is
done first, so any signals sent while suspended will be caught in the
subsequent signal check.

Reviewed by:	kib
Approved by:	kib (mentor)
MFC after:	2 weeks
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D9530