btrfs: extent-tree: Add lockdep assert when updating space info

Just add a safe net for btrfs_space_info member updating.

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