dm integrity: decouple common code in dm_integrity_map_continue()

Decouple how dm_integrity_map_continue() responds to being out of free
sectors and when add_new_range() fails.

This has no functional change, but helps prepare for the next commit.

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