Windows: Improvements to background fetch processing

Windows: Improvements to background fetch processing

Log offset and length in cm_BkgPrefetch()

Convert mxheld to rwheld in cm_BkgPrefetch() now that cm_scache_t
objects use rwlocks.

Do not clear CM_SCACHEFLAG_PREFETCHING from within the error
returns from cm_CheckFetchRange().  Let the caller decide if
that is appropriate.

Add CM_BUF_CMBKGFETCH cm_buf_t cmFlag to make it possible to
quickly detect if a background fetch operation has already
been queued for a particular cm_buf_t data range.

LICENSE MIT

Change-Id: I4ac9a2f84ddd64cba86612d7a2abe849bd0bec0b
Reviewed-on: http://gerrit.openafs.org/827
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>