btrfs: warn if extent buffer mapping crosses a page boundary in csum_tree_block

Since commit d2e174d5 ("btrfs: document extent mapping assumptions in
checksum") we have a comment in place why map_private_extent_buffer()
can't return 1 in the csum_tree_block() case.

Make this a bit more explicit and WARN_ON() in case this this assumption
breaks.
Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -285,7 +285,7 @@ static int csum_tree_block(struct extent_buffer *buf, u8 *result)
 		 */
 		err = map_private_extent_buffer(buf, offset, 32,
 					&kaddr, &map_start, &map_len);
-		if (err)
+		if (WARN_ON(err))
 			return err;
 		cur_len = min(len, map_len - (offset - map_start));
 		crc = btrfs_csum_data(kaddr + offset - map_start,