btrfs: add data_race() in btrfs_account_ro_block_groups_free_space()

btrfs: add data_race() in btrfs_account_ro_block_groups_free_space()

Surround the intentional empty list check with the data_race() annotation
so that tools like KCSAN don't report a data race. The race is intentional
as it's harmless and we want to avoid lock contention of the space_info
since its lock is heavily used (space reservation, space flushing, extent
allocation and deallocation, etc).

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>