kernel - dm - Add support for BUF_CMD_FLUSH

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.