blk-settings: round down io_opt to physical_block_size

There was a bug report [1] where the user got a warning alignment
inconsistency. The user has optimal I/O 16776704 (0xFFFE00) and physical
block size 4096. Note that the optimal I/O size may be set by the DMA
engines or SCSI controllers and they have no knowledge about the disks
attached to them, so the situation with optimal I/O not aligned to
physical block size may happen.

This commit makes blk_validate_limits round down optimal I/O size to the
physical block size of the block device.

Closes: https://lore.kernel.org/dm-devel/1426ad71-79b4-4062-b2bf-84278be66a5d@redhat.com/T/

 [1]
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: a2363464 ("block: take io_opt and io_min into account for max_sectors")
Cc: stable@vger.kernel.org	# v6.11+
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/3dc0014b-9690-dc38-81c9-4a316a2d4fb2@redhat.com


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