btrfs-progs: check/lowmem: fix INODE_REF repair

[BUG]
Although lowmem has the logical to repair one missing
INODE_REF/DIR_INDEX/DIR_ITEM, there is a catch in missing INODE_REF.

If we're checking an DIR_ITEM (which we normally hits first), and failed
to find the INODE_REF, we are in a situation where there is no @index to
delete the original DIR_INDEX/DIR_ITEM pair.

This can cause further damage to the fs, without really improving
anything.

[CAUSE]
There is a minimal example where there is missing INODE_ITEM:

	item 0 key (256 INODE_ITEM 0) itemoff 16123 itemsize 160
		generation 3 transid 9 size 12 nbytes 16384
		block group 0 mode 40755 links 1 uid 0 gid 0 rdev 0
		sequence 1 flags 0x0(none)
	item 1 key (256 INODE_REF 256) itemoff 16111 itemsize 12
		index 0 namelen 2 name: ..
	item 2 key (256 DIR_ITEM 496027801) itemoff 16075 itemsize 36
		location key (257 INODE_ITEM 0) type FILE
		transid 9 data_len 0 name_len 6
		name: foobar
	item 3 key (256 DIR_INDEX 2) itemoff 16039 itemsize 36
		location key (257 INODE_ITEM 0) type FILE
		transid 9 data_len 0 name_len 6
		name: foobar
	item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
		generation 9 transid 9 size 0 nbytes 0
		block group 0 mode 100644 links 1 uid 0 gid 0 rdev 0
		sequence 1 flags 0x0(none)

For above case, if we're check the DIR_ITEM, we can find the INODE_ITEM
but not the INODE_REF.

So we need to repair it, but at this stage we haven't checked DIR_INDEX,
this means we have no idea which index we should delete, leaving the
default @index as (-1).

If we call btrfs_unlink() with that (-1) as index, it will not delete
the (256 DIR_INDEX 2) one, then re-add a link using -1 as index, causing
more damage.

[FIX]
Before calling btrfs_unlink(), do an extra check on if we're called from
DIR_ITEM checks (aka, @index is -1) and we only detected a missing
INODE_REF yet.

If so, do find_dir_index() call to determine the DIR_INDEX, if that
failed it means we have missing both DIR_INDEX and INODE_REF, thus
should remove the lone DIR_ITEM instead.

With this enhancement, lowmem mode can properly fix the missing
INODE_REF by adding it back.

Signed-off-by: Qu Wenruo <wqu@suse.com>

Author: adam900710

check/mode-lowmem.c

@@ -1015,6 +1015,17 @@ static int repair_ternary_lowmem(struct btrfs_root *root, u64 dir_ino, u64 ino,
 	int stage = 0;
 	int ret = 0;
 
+	/*
+	 * We miss an INODE_REF, and we're checking DIR_ITEM and hasn't yet
+	 * find the DIR_INDEX, thus there is no reliable index.
+	 * Try to locate one, this can be slow as we need to locate the DIR_INDEX
+	 * item from the directory.
+	 */
+	if (index == (u64)-1 && (err & INODE_REF_MISSING)) {
+		ret = find_dir_index(root, dir_ino, ino, &index, name, name_len, filetype);
+		if (ret < 0)
+			err |= DIR_INDEX_MISSING;
+	}
 	/*
 	 * stage shall be one of following valild values:
 	 *	0: Fine, nothing to do.