Windows: Reset CM_VOLUMEFLAG_RO_SIZE_VALID flag

The CM_VOLUMEFLAG_RO_SIZE_VALID flag was being reset using the
wrong field which resulted in the flag never being cleared and
the correct volume size not being reported.

Change-Id: Ic6cefcefcd2c0ad20ad79a98cf0603f09efabcba
Reviewed-on: http://gerrit.openafs.org/9805
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>