Btrfs: don't lookup csums for prealloc extents

The tree logging stuff was looking up csums to copy over for prealloc
extents which is just work we don't need to be doing.  Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/tree-log.c

@@ -3011,8 +3011,7 @@ static noinline int copy_items(struct btrfs_trans_handle *trans,
 				continue;
 
 			found_type = btrfs_file_extent_type(src, extent);
-			if (found_type == BTRFS_FILE_EXTENT_REG ||
-			    found_type == BTRFS_FILE_EXTENT_PREALLOC) {
+			if (found_type == BTRFS_FILE_EXTENT_REG) {
 				u64 ds, dl, cs, cl;
 				ds = btrfs_file_extent_disk_bytenr(src,
 								extent);