btrfs: tree-checker: fix misleading root drop_level error message

Fix tree-checker error message to report "invalid root drop_level"
instead of the misleading "invalid root level".

Fixes: 259ee7754b67 ("btrfs: tree-checker: Add ROOT_ITEM check")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>