Windows: avoid race when writing mountPointString

Windows: avoid race when writing mountPointString

cm_GetData() drops the cm_scache_t rw lock which permits other
threads to access the data while it is in an inconsistent state.
Avoid the race by using a stack allocated temporary buffer to
receive the data from cm_GetData().  Only copy the data into
the mountPointStringp buffer under the rwlock.

Change-Id: Ica853976b1094be1087e49c22d878f8ae7fca03a
Reviewed-on: http://gerrit.openafs.org/4498
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>