darwin: don't process incoming packets during rx shutdown

we have a chicken and egg problem because the kpi doesn't
let us kill the upcall before we close a socket. a more correct
fix (on master) uses atomics, but for 1.6 this is a minimally-invasive
fix, macos only and strictly better than before.

FIXES 131577

Change-Id: I6003679f709284ae28f51bf5de8deff873346819
Reviewed-on: http://gerrit.openafs.org/8937
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Paul Smeddle <paul.smeddle@gmail.com>