btrfs: allow block group background reclaim for non-zoned filesystems

This will allow us to set a threshold for block groups to be
automatically relocated even if we don't have zoned devices.

We have found this feature invaluable at Facebook due to how our
workload interacts with the allocator.  We have been using this in
production for months with only a single problem that has already been
fixed.

Tested-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>