Windows: Do not issue RXAFS change RPCs on known RO volumes

Windows: Do not issue RXAFS change RPCs on known RO volumes

If the cm_scache_t is known to be on a RO volume, do not permit
RXAFS_xxx RPCs that would attempt to make a change to the volume
to be issued to the file server.  Instead, return CM_ERROR_READONLY
immediately.   This avoids triggering the abort threshold for
the current connection on the file server.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/2950
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 7f742f45a2aaa9dcb400aa1dc35ce617876fe7fd)
Change-Id: I7e58f35956be8f1acd4ebfbd407ddde6b8bcb136
Reviewed-on: http://gerrit.openafs.org/3057
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>