Windows: AFSExamineVolume drop TreeLock if waiters

After each call to AFSExamineObject drop the ObjectInfoTree.TreeLock
if there are threads waiting for access.  The garbage collection process
should not delay real work.

Change-Id: I2bd009d71b534d92d7e1b321b6db2204643e4666
Reviewed-on: http://gerrit.openafs.org/9786
Reviewed-by: Rod Widdowson <rdw@steadingsoftware.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>