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 <j-nomura@ce.jp.nec.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>