Windows: Add flag BUF_GET_FLAG_BUFCREATE_LOCKED

Windows: Add flag BUF_GET_FLAG_BUFCREATE_LOCKED

Modify the buf_Get() and buf_GetNewLocked() interfaces to
permit the cm_scache.bufCreateLocked lock to be held prior to
calling to buf_Get().  Holding the cm_scache.bufCreateLocked lock
before the buf_Get() call prevents a race with another thread
that attempts to set the file size.

Change-Id: I54b13c83801a4d9a2078552d5be7ada6f4607850
Reviewed-on: http://gerrit.openafs.org/9209
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>