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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273811 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f