kernel - dm - Add support for BUF_CMD_FLUSH
* Change the nestiobuf_*() code to count the number of sub-bios issued
instead of counting the resid. This allows the nestiobuf_*() API to
be used to BUF_CMD_FLUSH.
* Change the nestiobuf_*() API. The nestiobuf_start() command is now
nestiobuf_add(). Add a nestiobuf_init(mbio) and nestiobuf_start(mbio)
wrapper around the adds.
* Explicitly check for allowed BUF_CMD_* commands in device-mapper.c
* Add a bypass to device-mapper.c to cause certain commands to be sent
to all tbl's (BUF_CMD_FLUSH).
* Add explicit suppor for BUF_CMD_FLUSH to dm_target_stripe.c
* Change b_resid loading semantics to load upon completion of the
I/O.