dm: reduce code duplication in __map_bio

Error path code (for handling DM_MAPIO_REQUEUE and DM_MAPIO_KILL) is
effectively identical.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>