block: add call to split trace point

There is a split tracepoint that is supposed to be called when
bio is splitted, and it was called in bio_split function until
commit 4b1faf93

 ("block: Kill bio_pair_split()").
But now, no one reports splits, so this patch adds call to
trace_block_split back in blk_queue_split right after split.

Signed-off-by: Mike Krinkin <krinkin.m.u@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>