btrfs-progs: check: fix DIR_ITEM checking in lowmem

btrfs-progs: check: fix DIR_ITEM checking in lowmem

When checking the validity of a DIR_ITEM item the index variable is
explicitly set to -1 so that the index check in find_inode_ref() is
ignored. This is necessary due to possible name collisions in DIR_ITEMS
(i.e. multiple files with the same crc32c for their names, resulting in
the identical key->offset). Currently the code is broken due to index
being set to -1 at the beginning of check_dir_item and subsequently
reset to the index found in find_inode_ref. On subsequent iterations of
the while loop in check_dir_items we are going to erroneously perform
index checking, since index is not -1 but whatever the index value of
the last handled INODE_REF ITEM.

Without this patch check in lowmem mode produces the following false
warnings positivess:

ERROR: root 5 INODE REF[1991456, 256] name 5ab4e28b~~~~~~~~QBXUT2GBJDRT5CB6ZWAJVDHK filetype 1 missing

But the items for this name are in fact correct:

------------------SNIP------------------------
item 13 key (256 DIR_ITEM 4227063046) itemoff 2945 itemsize 140
	location key (220890 INODE_ITEM 0) type FILE
	transid 2278 data_len 0 name_len 40
	name: 5ab4e1bb~~~~~~~~65KNTAWVJ5VD4SV2R3BKFCXL
	location key (1991456 INODE_ITEM 0) type FILE
	transid 2285 data_len 0 name_len 40
	name: 5ab4e28b~~~~~~~~QBXUT2GBJDRT5CB6ZWAJVDHK

item 21 key (1991456 INODE_REF 256) itemoff 2104 itemsize 50
	index 1934146 namelen 40 name: 5ab4e28b~~~~~~~~QBXUT2GBJDRT5CB6ZWAJVDHK
-------------------SNIP-------------------------

Fix this by moving the code setting index at the beginning of the while
loop. This ensure that for each item in DIR_ITEM we have index set
correctly.

Fixes: 564901eac7a4 ("btrfs-progs: check: introduce print_dir_item_err()")
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Su Yue <suy.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>