btrfs-progs: check: repair dir_item and inode_ref in lowmem mode

btrfs-progs: check: repair dir_item and inode_ref in lowmem mode

Introduce repair_ternary_lowmem() to repair dir_item, dir_index
and inode_ref.
If two of the three are missing or mismatched, call btrfs_unlink() to
delete the existing one.
If one of three is missing or mismatched, call btrfs_add_link() to
add the missing one.

repair_dir_item() inserts an inode item corresponding to location in the
dir item if error contains INODE_ITEM_MISSING.
Also, it calls repair_ternary_lowmem() to repair relationship of
dir_item, dir_index and inode_ref.

check_inode_ref() calls repair_ternary_item() to fix up errors.

Signed-off-by: Su Yue <suy.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>