Windows: cm_MergeStatus avoid lock recursion

It is possible for cm_MergeStatus() to be called while the
cm_buf_t.mx is already held.  If it is a panic occurs.  Test for
refcount == 0 before acquiring the lock in addition to afterwards.
If the refcount is not zero, then we do not need to acquire the
lock in any case.

Change-Id: I1b73a03f4745e524d7fdf8f9b231b420895ff0fa
Reviewed-on: http://gerrit.openafs.org/10297
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>