tty: tty_buffer: use __tty_insert_flip_string_flags() in tty_insert_flip_char()

tty: tty_buffer: use __tty_insert_flip_string_flags() in tty_insert_flip_char()

Use __tty_insert_flip_string_flags() for the slow path of
tty_insert_flip_char(). The former is generic enough, so there is no
reason to reimplement the injection once again.

So now we have a single function stuffing into tty buffers.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230816105530.3335-8-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>