Windows: Decrement Fcb OpenHandleCount while locked

AFSCleanup performs tests on the Fcb Open Handle Count to determine
when to perform final cleanup tasks on the last handle close.  The
test is protected by holding the Fcb Resource.  If the Open Handle
Count is decremented after dropping the Resource, it creates a
race with other threads that might be blocked entering AFSCleanup
to close their handle on the same object.

Reviewed-on: http://gerrit.openafs.org/8863
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit e297a00dfca21103672ab97145895cabf97112a8)

Change-Id: I414d1514bf4b201960aef9286695c28647a439f6
Reviewed-on: http://gerrit.openafs.org/9012
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>