Windows: Update Bulk I/O Descriptor

Update the cm_bulkIO_t and associated functions such that
the 'reserved' field is no longer a boolean indicated whether
or not buffers have been reserved but instead becomes a count
of the number of buffers that have been reserved.

buf_TryReserveBuffers is modified to return an afs_uint64
count of the number of buffers reserved instead of a boolean.

cm_SetupStoreBIOD, cm_SetupFetchBIOD, and cm_ReleaseBIOD altered
to store the reserved buffer count and use it when freeing the
BIOD.

Prior to this change it was not possible to reserve a count other
than the number of buffers it takes to store one chunkSize of data.

Change-Id: I78d71c77ffddd7f34278314fcfe9ee568b835a8b
Reviewed-on: http://gerrit.openafs.org/8951
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
Windows: rename 'rbytes' to 'rxbytes' for clarity

Throughout cm_dcache.c, the various 'rbytes' represents the number
of bytes to be read from the next rx_Read or rx_Readv call.
Rename the variable to 'rxbytes' to improve clarity.

Change-Id: Iffd8dd397c0e6ab4cc44019b7f937a80f5d492d2
Reviewed-on: http://gerrit.openafs.org/8952
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>