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

Change-Id: I9c917e60277d281e32e4609d89b541803824251f
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>