block-map: add __GFP_ZERO flag for alloc_page in function bio_copy_kern

Add __GFP_ZERO flag for alloc_page in function bio_copy_kern to initialize
the buffer of a bio.

Signed-off-by: Haimin Zhang <[e-mail]>
Reviewed-by: Chaitanya Kulkarni <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-map.c

···
 		if (bytes > len)
 			bytes = len;
 
-		page = alloc_page(GFP_NOIO | gfp_mask);
+		page = alloc_page(GFP_NOIO | __GFP_ZERO | gfp_mask);
 		if (!page)
 			goto cleanup;