darwin: stop processing upcalls once rx shutdown starts

darwin: stop processing upcalls once rx shutdown starts

we have a chicken and egg. can't stop upcall without
killing socket; can't kill socket while rx might try to transmit on it.
cheat, and if rx is shut down, don't process things upcall receives.

FIXES 131577

Change-Id: I6b72f6e7aefb7937b768dcb9c0a83d0ee3317bfa
Reviewed-on: http://gerrit.openafs.org/8931
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>