dm exception store: refactor zero_area

Use a separate buffer for writing zeroes to the on-disk snapshot
exception store, make the updating of ps->current_area explicit and
refactor the code in preparation for the fix in the next patch.

No functional change.

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@kernel.org