Windows: add lock assertions to cm_buf.c

Windows: add lock assertions to cm_buf.c

add lock assertions to cm_buf.c in hopes of detecting why
periodic buf_scacheHashTable corruption is being detected.

Change-Id: Ib93ac7d449c7f2cb065219981f16ba319a9273b1
Reviewed-on: http://gerrit.openafs.org/4659
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-on: http://gerrit.openafs.org/4674