tty: tty_buffer: use struct_size() in tty_buffer_alloc()

Now, that tty_buffer::data has the right type, use struct_size() for
size calculation. struct_size() makes the code less error-prone and more
readable.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230816105530.3335-3-jirislaby@kernel.org


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