dm: use dm_target_offset macro

Use new dm_target_offset() macro to avoid most references to ti->begin
in dm targets.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --cc drivers/md/dm-crypt.c
Simple merge

diff --cc drivers/md/dm-delay.c
Simple merge

diff --cc drivers/md/dm-linear.c
Simple merge

diff --cc drivers/md/dm-raid1.c
Simple merge

diff --cc drivers/md/dm-stripe.c
Simple merge