btrfs-progs: check: introduce low memory mode

btrfs-progs: check: introduce low memory mode

Introduce a new fsck mode: low memory mode.

Old btrfsck is working efficiently but uses some memory for each extent
item.  This method will ensure extents are only iterated once at
extent/chunk tree check process.

But since it uses some memory for each extent item, for a large fs with
several TB metadata, this can easily eat up memory and cause OOM.

To handle such limitation and improve scalability, the new low-memory
mode will not use any heap memory to record which extent is checked.
Instead it will use extent backref to avoid most of uneeded checks on
shared fs/subvolume tree blocks.
And with the use forward and backward reference cross check, we can also
ensure every tree block is at least checked once.

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>