btrfs: convert btrfs_block_group::needs_free_space to runtime flag

[ Upstream commit 0d7764ff ]

We already have flags in block group to track various status bits,
convert needs_free_space as well and reduce size of btrfs_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>