We must reset the zones of a deleted unused block group to rewind the
zones' write pointers to the zones' start.

To do this, we can use the DISCARD_SYNC code to do the reset when the
filesystem is running on zoned devices.
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NNaohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>