btrfs: convert btrfs_block_group::seq_zone to runtime flag

[ Upstream commit 961f5b8b ]

In zoned mode the sequential status of zone can be also tracked in the
runtime flags of block group.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 0657b20c ("btrfs: fix use-after-free of new block group that became unused")
Signed-off-by: Sasha Levin <sashal@kernel.org>