btrfs: tree-checker: reject BTRFS_FT_UNKNOWN dir type

[REPORT]
There is a bug report that kernel is rejecting a mismatching inode mode
and its dir item:

  [ 1881.553937] BTRFS critical (device dm-0): inode mode mismatch with
  dir: inode mode=040700 btrfs type=2 dir type=0

[CAUSE]
It looks like the inode mode is correct, while the dir item type
0 is BTRFS_FT_UNKNOWN, which should not be generated by btrfs at all.

This may be caused by a memory bit flip.

[ENHANCEMENT]
Although tree-checker is not able to do any cross-leaf verification, for
this particular case we can at least reject any dir type with
BTRFS_FT_UNKNOWN.

So here we enhance the dir type check from [0, BTRFS_FT_MAX), to
(0, BTRFS_FT_MAX).
Although the existing corruption can not be fixed just by such enhanced
checking, it should prevent the same 0x2->0x0 bitflip for dir type to
reach disk in the future.
Reported-by: Kota <nospam@kota.moe>
Link: https://lore.kernel.org/linux-btrfs/CACsxjPYnQF9ZF-0OhH16dAx50=BXXOcP74MxBc3BG+xae4vTTw@mail.gmail.com/
CC: stable@vger.kernel.org # 5.4+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-checker.c

@@ -569,9 +569,10 @@ static int check_dir_item(struct extent_buffer *leaf,
 
 		/* dir type check */
 		dir_type = btrfs_dir_ftype(leaf, di);
-		if (unlikely(dir_type >= BTRFS_FT_MAX)) {
+		if (unlikely(dir_type <= BTRFS_FT_UNKNOWN ||
+			     dir_type >= BTRFS_FT_MAX)) {
 			dir_item_err(leaf, slot,
-			"invalid dir item type, have %u expect [0, %u)",
+			"invalid dir item type, have %u expect (0, %u)",
 				dir_type, BTRFS_FT_MAX);
 			return -EUCLEAN;
 		}