macos: get more packets if requested

analogous to eead07418f13cd87a90770f03ea5118c546d8f1a for ukernel,
this is for macos' upcall procedure

Reviewed-on: http://gerrit.openafs.org/7788
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 33a761ebd11e33b5e111e67204218e8f67bef431)

Change-Id: I1b59c802717393758370c376f95569467dc61049
Reviewed-on: http://gerrit.openafs.org/9747
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>