Windows: cm_SetupFetchBIOD reserving type

Change type of 'reserving' to afs_uint64 to match the return type
of buf_TryReserveBuffers().

Change-Id: I8aaac5a763ec55acada2702b898a9bcd50f82ed0
Reviewed-on: http://gerrit.openafs.org/9062
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>