btrfs: tree-checker: adjust error code for header level check

The whole tree checker returns EUCLEAN, except the one check in
btrfs_verify_level_key(). This was inherited from the function that was
moved from disk-io.c in 2cac5af ("btrfs: move
btrfs_verify_level_key into tree-checker.c") but this should be unified
with the rest.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/tree-checker.c

@@ -2235,7 +2235,7 @@ int btrfs_verify_level_key(struct extent_buffer *eb,
 		btrfs_err(fs_info,
 "tree level mismatch detected, bytenr=%llu level expected=%u has=%u",
 			  eb->start, check->level, found_level);
-		return -EIO;
+		return -EUCLEAN;
 	}
 
 	if (!check->has_first_key)