Windows: Do not leak activeRPC count

If a callback race has been lost cm_MergeStatus is not executed.
In that case either the activeRPC count should not be incremented
or must be decremented to indicate that the current call has been
completed.

Reviewed-on: http://gerrit.openafs.org/10218
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 28de757a65679bff74fb34e65d8cd78cb36d0fc9)

Change-Id: Ic3aeb797fa745cb65974890caa6a2615ec776d51
Reviewed-on: http://gerrit.openafs.org/10223
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>