iomap: use __bio_add_page in iomap_dio_zero

We don't need any merging logic, and this also replaces a BUG_ON with a
WARN_ON_ONCE inside __bio_add_page for the impossible overflow condition.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>