iomap: refactor iomap_dio_actor

Split the function up into two helpers for the bio based I/O and hole
case, and a small helper to call the two.  This separates the code a
little better in preparation for supporting I/O to inline data.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>