tty: tty_buffer: switch data type to u8

tty: tty_buffer: switch data type to u8

There is no reason to have tty_buffer::data typed as unsigned long.
Switch to u8, but preserve the ulong alignment using __aligned.

This allows for the cast removal from char_buf_ptr(). And for use of
struct_size() in the allocation in tty_buffer_alloc() -- in the next
patch.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230816105530.3335-2-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>