Btrfs: fix kfree on list_head in btrfs_lookup_csums_range error cleanup

commit 6e5aafb2 upstream.

If we hit any errors in btrfs_lookup_csums_range, we'll loop through all
the csums we allocate and free them.  But the code was using list_entry
incorrectly, and ended up trying to free the on-stack list_head instead.

This bug came from commit 0678b618

btrfs: Don't BUG_ON kzalloc error in btrfs_lookup_csums_range()
Signed-off-by: Chris Mason <clm@fb.com>
Reported-by: Erik Berg <btrfs@slipsprogrammoer.no>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

fs/btrfs/file-item.c

@@ -423,7 +423,7 @@ int btrfs_lookup_csums_range(struct btrfs_root *root, u64 start, u64 end,
 	ret = 0;
 fail:
 	while (ret < 0 && !list_empty(&tmplist)) {
-		sums = list_entry(&tmplist, struct btrfs_ordered_sum, list);
+		sums = list_entry(tmplist.next, struct btrfs_ordered_sum, list);
 		list_del(&sums->list);
 		kfree(sums);
 	}