tty: tty_buffer: make all offsets unsigned

All these are supposed/expected to be unsigned as they are either counts
or offsets. So switch to unsigned for clarity.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-21-jirislaby@kernel.org


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