Various small bits that make SCSI work well with REQ_BLOCK_PC.

o Use ->errors as the scsi status byte for REQ_BLOCK_PC

o Always call end_io completion, even if 0 sectors, as long as the
  status is good. Otherwise we risk hanging this device if a
  REQ_BLOCK_PC user command didn't specify a transfer size for a command
  that did.

o Remove bouncing checks in scsi_merge for REQ_BLOCK_PC, bio_map_user()
  correctly bounces pages now.

o Decrement req->data_len, it's our residual data count.

o sr/sd: set right transfer and underflow size.