btrfs: change the bg_reclaim_threshold valid region from 0 to 100

For the non-zoned case we may want to set the threshold for reclaim to
something below 50%.  Change the acceptable threshold from 50-100 to
0-100.

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>