NetBSD: fix rx_kmutex's CV_WAIT_SIG macro

Change-Id: I6550f5c71bafe03290c1dbda545ff3feea01a805
Reviewed-on: http://gerrit.openafs.org/11088
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>