Windows: IsSpaceAvail lock order violation

cm_IsSpaceAvailable() obtains the cm_scache.rw lock of the volume
root directory.  Therefore it is a lock order violation to call the
function while any other cm_scache.rw lock is held belonging to an
object in the same volume.   vnode 1 is always less than any other
vnode value.

Change-Id: Id34591b6ccec8d7e8e0fe48e3357c991cd99acfb
Reviewed-on: http://gerrit.openafs.org/9552
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>