btrfs: add unlikely to critical error in btrfs_extend_item()

It's not expected to get a data size less than the leaf's free space,
which would lead to a leaf dump and BUG(), so tag the if statement's
expression as unlikely, hinting the compiler to potentially generate
better code.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/ctree.c

@@ -4132,7 +4132,7 @@ void btrfs_extend_item(struct btrfs_trans_handle *trans,
 	nritems = btrfs_header_nritems(leaf);
 	data_end = leaf_data_end(leaf);
 
-	if (btrfs_leaf_free_space(leaf) < data_size) {
+	if (unlikely(btrfs_leaf_free_space(leaf) < data_size)) {
 		btrfs_print_leaf(leaf);
 		BUG();
 	}