dm integrity: add ic->start in get_data_sector()

A small refactoring.  Add the variable ic->start to the result
returned by get_data_sector() and not in the callers.  This is a
prerequisite for the commit that adds the ability to use an external
metadata device.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>