block: add @req to bio_{front|back}_merge tracepoints

bio_{front|back}_merge tracepoints report a bio merging into an
existing request but didn't specify which request the bio is being
merged into.  Add @req to it.  This makes it impossible to share the
event template with block_bio_queue - split it out.

@req isn't used or exported to userland at this point and there is no
userland visible behavior change.  Later changes will make use of the
extra parameter.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>