Btrfs: fix hole check in log_one_extent

I added an assert to make sure we were looking up aligned offsets for csums and
I tripped it when running xfstests.  This is because log_one_extent was checking
if block_start == 0 for a hole instead of EXTENT_MAP_HOLE.  This worked out fine
in practice it seems, but it adds a lot of extra work that is uneeded.  With
this fix I'm no longer tripping my assert.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/tree-log.c

@@ -3373,7 +3373,7 @@ static int log_one_extent(struct btrfs_trans_handle *trans,
 		btrfs_set_token_file_extent_type(leaf, fi,
 						 BTRFS_FILE_EXTENT_REG,
 						 &token);
-		if (em->block_start == 0)
+		if (em->block_start == EXTENT_MAP_HOLE)
 			skip_csum = true;
 	}