block: don't bother checking the data direction for merges

Because it already is encoded in the opcode.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20241119161157.1328171-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>