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>

fs/iomap.c

@@ -845,8 +845,7 @@ iomap_dio_zero(struct iomap_dio *dio, struct iomap *iomap, loff_t pos,
 	bio->bi_end_io = iomap_dio_bio_end_io;
 
 	get_page(page);
-	if (bio_add_page(bio, page, len, 0) != len)
-		BUG();
+	__bio_add_page(bio, page, len, 0);
 	bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC | REQ_IDLE);
 
 	atomic_inc(&dio->ref);