rping: make sure that the CQ event thread can never poll a CQ after it
has been destroyed.

Submitted by:	Hariprasad at Chelsio dot com
Sponsored by:	Chelsio Communications
Add discovery-filter.  This makes it possible to restrict which targets
are returned during discovery based on initiator portal, name, and CHAP
credentials.

Reviewed by:	mav@
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Simplify code; no functional changes.

MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Merge remote-tracking branch 'freebsd/master' into hardened/current/master
Merge remote-tracking branch 'origin/hardened/current/master' into hardened/current/pie