btrfs: use explicit initializer for seq_elem

Using {} as initializer for struct seq_elem does not properly initialize
the list_head member, but it currently works because it gets set through
btrfs_get_tree_mod_seq if 'seq' is 0.

Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/ioctl.c

@@ -3039,7 +3039,7 @@ out:
 static int check_ref(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 		     u64 disko)
 {
-	struct seq_list tree_mod_seq_elem = {};
+	struct seq_list tree_mod_seq_elem = SEQ_LIST_INIT(tree_mod_seq_elem);
 	struct ulist *roots;
 	struct ulist_iterator uiter;
 	struct ulist_node *root_node = NULL;