macos: lock module against unload during shutdown

macos: lock module against unload during shutdown

during shutdown, once umount finished, it was possible to
unload the module before shutdown completed. fix that.

Reviewed-on: http://gerrit.openafs.org/7245
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit a08f9baa7620a6643f12f0b847c2bbab41a0b255)

Change-Id: I6467b9ead03262a6ec8b0de673d2eeb664df1aef
Reviewed-on: http://gerrit.openafs.org/7484
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>