btrfs: sysfs: relax bg_reclaim_threshold for debugging purposes

Currently, /sys/fs/btrfs/<UUID>/bg_reclaim_threshold is limited to 0
(disable) or [50 .. 100]%, so we need to fill 50% of a device to start the
auto reclaim process. It is cumbersome to do so when we want to shake out
possible race issues of normal write vs reclaim.

Relax the threshold check under the BTRFS_DEBUG option.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>