rx: Helper function for decrementing conn refcnt

rx: Helper function for decrementing conn refcnt

The code to lock the reference count mutex, reduce the connection
reference count, then unlock the mutex, is duplicated many times
throughout rx.c. Replace all of these multiple copies with a single
inline function.

Change-Id: I7adb460019d7434b32a16250cc026be667e7c55d
Reviewed-on: http://gerrit.openafs.org/6218
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementix.org>