Windows: buf_CleanVnode avoid lock contention

In the middle of the big loop do not obtain buf_globalLock unless
cm_buf.qFlags & CM_BUF_QREDIR is non-zero.  buf_globalLock is heavily
contested and any reduction in contention is an improvement.
CM_BUF_QREDIR is never set when 'cm_directIO' is true (now the default)
so this is a huge win.

Change-Id: I0b150bf77a0f1d748f94026d3b74bc7d90e7c3c6
Reviewed-on: http://gerrit.openafs.org/9636
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>