btrfs-progs: check: introduce function to speed up fs tree check

btrfs-progs: check: introduce function to speed up fs tree check

Introduce function should_check() to reduced duplicated tree block check
for fs/subvolume tree.

The idea is, we only check the fs/subvolue tree block if we have the
lowest referencer rootid, according to extent tree.

In that case, we can skip a lot of fs/subvolume tree block checks if
there are a lot of snapshots.

Although we will do a lot of extent tree searches for it.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>