tty: tty_buffer: warn if losing flags in __tty_insert_flip_string_flags()

And add a WARN_ON_ONCE(need_flags) to make sure we are not losing flags
in __tty_insert_flip_string_flags().

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230816105530.3335-5-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>