Windows: FSCTL_IS_PATHNAME_VALID return success

Windows: FSCTL_IS_PATHNAME_VALID return success

Windows NT, Windows 2000, Windows XP, Windows Server 2003, Windows Vista,
Windows Server 2008, Windows 7, and Windows Server 2008 R2 support the
FSCTL_IS_PATHNAME_VALID Request (section 2.3.21) and return STATUS_SUCCESS
whenever this request is invoked.  We will do the same.

Change-Id: I432c8a41e1102648faaa3d8c0f3b8c36ea89bfc8
Reviewed-on: http://gerrit.openafs.org/5827
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>