block: Add nr_bios to block_rq_remap tracepoint

block: Add nr_bios to block_rq_remap tracepoint

Adding the number of bios in a remapped request to 'block_rq_remap'
tracepoint.

Request remapper clones bios in a request to track the completion
status of each bio. So the number of bios can be useful information
for investigation.

Related discussions:
  http://www.redhat.com/archives/dm-devel/2013-August/msg00084.html
  http://www.redhat.com/archives/dm-devel/2013-September/msg00024.html

Signed-off-by: Jun'ichi Nomura <[email protected]>
Acked-by: Mike Snitzer <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>