tty: tty_buffer: switch insert functions to size_t

tty: tty_buffer: switch insert functions to size_t

All the functions accept size_t as a size argument. They finally return
the same size (or less). It is quite unexpected that they return a
signed value and can confuse users to check for negative values.

Instead, return the same size_t as accepted to make clear we return
values >= 0, where zero in fact means failure.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230816105530.3335-6-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>