dm cache: set minimum_io_size to cache's data block size

Before, if the block layer's limit stacking didn't establish an
optimal_io_size that was compatible with the cache's data block size
we'd set optimal_io_size to the data block size and minimum_io_size to 0
(which the block layer adjusts to be physical_block_size).

Update cache_io_hints() to set both minimum_io_size and optimal_io_size
to the cache's data block size. This fixes an issue where mkfs.xfs
would create more XFS Allocation Groups on cache volumes than on a
normal linear LV of comparable size.

Signed-off-by: Mike Snitzer <[e-mail]>

drivers/md/dm-cache-target.c

···
 	 */
 	if (io_opt_sectors < cache->sectors_per_block ||
 	    do_div(io_opt_sectors, cache->sectors_per_block)) {
-		blk_limits_io_min(limits, 0);
+		blk_limits_io_min(limits, cache->sectors_per_block << SECTOR_SHIFT);
 		blk_limits_io_opt(limits, cache->sectors_per_block << SECTOR_SHIFT);
 	}
 	set_discard_limits(cache, limits);