btrfs: annotate block group access with data_race() when sorting for reclaim

When sorting the block group list for reclaim we are using a block group's
used bytes counter without taking the block group's spinlock, so we can
race with a concurrent task updating it (at btrfs_update_block_group()),
which makes tools like KCSAN unhappy and report a race.

Since the sorting is not strictly needed from a functional perspective
and such races should rarely cause any ordering changes (only load/store
tearing could cause them), not to mention that after the sorting the
ordering may no longer be accurate due to concurrent allocations and
deallocations of extents in a block group, annotate the accesses to the
used counter with data_race() to silence KCSAN and similar tools.

Reviewed-by: Qu Wenruo <wqu@suse.com>
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>