tty: make char_buf_ptr()/flag_buf_ptr()'s offset unsigned

The offset is meant from the beginning of data, so unsigned. Make it as
such for clarity. All struct tty_buffer's members should be unsigned
too -- see the next patch.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-20-jirislaby@kernel.org


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