dm: cleanup of front padding calculation

Add two helper macros calculating the offset of bio in struct dm_io and
struct dm_target_io respectively.

Besides, simplify the front padding calculation in
dm_alloc_md_mempools().

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>