dm integrity: introduce rw_journal_sectors()

Introduce a function rw_journal_sectors() that takes sector and length
as its arguments instead of a section and the number of sections.

This functions will be used in further patches.

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