btrfs: zlib: use copy_page for full page copy

The copy_page helper may use an optimized version for full page copy
(eg. on s390 there's a special instruction for that), there's one more
left to convert.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/zlib.c

@@ -155,8 +155,8 @@ int zlib_compress_pages(struct list_head *ws, struct address_space *mapping,
 					in_page = find_get_page(mapping,
 								start >> PAGE_SHIFT);
 					data_in = kmap_local_page(in_page);
-					memcpy(workspace->buf + i * PAGE_SIZE,
-					       data_in, PAGE_SIZE);
+					copy_page(workspace->buf + i * PAGE_SIZE,
+						  data_in);
 					start += PAGE_SIZE;
 				}
 				workspace->strm.next_in = workspace->buf;