dm cache: fix some issues with the new discard range support

Commit 7ae34e77 ("dm cache: improve discard support") needed to also:
- discontinue having DM core split the discard bios on cache block
  boundaries
- calculate the cache's discard_nr_blocks relative to the determined
  discard_block_size rather than using oblock_to_dblock()
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-cache-target.c

@@ -2338,8 +2338,7 @@ static int cache_create(struct cache_args *ca, struct cache **result)
 	ti->num_discard_bios = 1;
 	ti->discards_supported = true;
 	ti->discard_zeroes_data_unsupported = true;
-	/* Discard bios must be split on a block boundary */
-	ti->split_discard_bios = true;
+	ti->split_discard_bios = false;
 
 	cache->features = ca->features;
 	ti->per_bio_data_size = get_per_bio_data_size(cache);
@@ -2440,7 +2439,8 @@ static int cache_create(struct cache_args *ca, struct cache **result)
 	cache->discard_block_size =
 		calculate_discard_block_size(cache->sectors_per_block,
 					     cache->origin_sectors);
-	cache->discard_nr_blocks = oblock_to_dblock(cache, cache->origin_blocks);
+	cache->discard_nr_blocks = to_dblock(dm_sector_div_up(cache->origin_sectors,
+							      cache->discard_block_size));
 	cache->discard_bitset = alloc_bitset(from_dblock(cache->discard_nr_blocks));
 	if (!cache->discard_bitset) {
 		*error = "could not allocate discard bitset";