bcachefs: Handle ec_buf not being page aligned when allocating bio

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io.c

@@ -461,7 +461,10 @@ static struct bio *bch2_write_bio_alloc(struct bch_fs *c,
 	struct bio *bio;
 	unsigned output_available =
 		min(wp->sectors_free << 9, src->bi_iter.bi_size);
-	unsigned pages = DIV_ROUND_UP(output_available, PAGE_SIZE);
+	unsigned pages = DIV_ROUND_UP(output_available +
+				      (buf
+				       ? ((unsigned long) buf & (PAGE_SIZE - 1))
+				       : 0), PAGE_SIZE);
 
 	bio = bio_alloc_bioset(NULL, pages, 0,
 			       GFP_NOIO, &c->bio_write);