ext4: fix wrong unit use in ext4_mb_clear_bb

Function ext4_issue_discard need count in cluster. Pass count_clusters
instead of count to fix the mismatch.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: stable@kernel.org
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20230603150327.3596033-11-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>