btrfs: get used bytes while holding lock at btrfs_reclaim_bgs_work()

btrfs: get used bytes while holding lock at btrfs_reclaim_bgs_work()

At btrfs_reclaim_bgs_work(), we are grabbing twice the used bytes counter
of the block group while not holding the block group's spinlock. This can
result in races, reported by KCSAN and similar tools, since a concurrent
task can be updating that counter while at btrfs_update_block_group().

So avoid these races by grabbing the counter in a critical section
delimited by the block group's spinlock after setting the block group to
RO mode. This also avoids using two different values of the counter in
case it changes in between each read. This silences KCSAN and is required
for the next patch in the series too.

Fixes: 243192b67649 ("btrfs: report reclaim stats in sysfs")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>