tty: use ssize_t for iterate_tty_read() returned type

tty_read() is supposed to return ssize_t. It takes the return value from
iterate_tty_read(). That currently returns int. On the top of that,
iterate_tty_write() already returns ssize_t. So switch
iterate_tty_read() to ssize_t too, so that all three are consistent.

This means 'i' in tty_read() changes its type too. And while changing
that, rename this generic 'i' to more dedicated 'ret'.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-25-jirislaby@kernel.org


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