Windows: optimize cm_BkgDaemon()

Windows: optimize cm_BkgDaemon()

In cm_BkgDaemon test for CM_SCACHEFLAG_DATASTORING before
cm_ServerAvailable() in order to avoid unnecessary work.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/3020
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 49e57b8c3b5e13f7ce48c6be5677827d970df5fa)
Change-Id: Ib33babf87605c8f93ee87e8f7c8084e213eedbc4
Reviewed-on: http://gerrit.openafs.org/3119
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>