Also rename blk_account_io_merge() into blk_account_io_merge_request() to
distinguish it from merging request and bio.

[hch: rebased]
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>