Windows: btree enumeration bulk stats

Each of the btree enumeration bulk stat operations include the
directory object in the bulk stat list.  If the only object in the
list is the directory object, do not perform the bulk stat rpc as
it just wastes time.  All of the required objects are already cached
with current callbacks.

Change-Id: Ic0fac50f36bdca1e79f48ca308c373a2b3152675
Reviewed-on: http://gerrit.openafs.org/9675
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
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>