dm thin metadata: fix __udivdi3 undefined on 32-bit

sector_div() is only viable for use with sector_t.
dm_block_t is typedef'd to uint64_t -- so use div_u64() instead.

Fixes: 3ab91828 ("dm thin metadata: try to avoid ever aborting transactions")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>