dm: remove useless loop in __split_and_process_bio

Remove useless "while" loop. If the condition ci.sector_count && !error is
true, we go to a branch that ends with "break". If this condition is
false, the "while" loop will not be executed again. So, the loop can't be
executed more than once.

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