Windows: cache readonly volume size information

Windows: cache readonly volume size information

Cache the volume size information for .readonly volumes which can
be reset when the volume callback is broken.  This reduces the number
of RXAFS_GetVolumeStatus RPC calls issues on .readonly volumes.

Change-Id: Ie0e63ca9082a004da71098e28df1315d42d364ff
Reviewed-on: http://gerrit.openafs.org/9681
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>