tty: switch size and count types in iterate_tty_read() to size_t

ld->ops->read() returns ssize_t. copy_to_iter() returns size_t. So
switch the variables ('size' and 'copied', respectively) to the
corresponding types.

This allows for use of min() in the next patch.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-26-jirislaby@kernel.org


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