btrfs: zoned: allow disabling of zone auto reclaim

Automatically reclaiming dirty zones might not always be desired for all
workloads, especially as there are currently still some rough edges with
the relocation code on zoned filesystems.

Allow disabling zone auto reclaim on a per filesystem basis by writing 0
as the threshold value.

Reviewed-by: Naohiro Aota <naohiro.aota@wdc.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>