mptcp: getsockopt: add support for IP_TOS

earlier patch added IP_TOS setsockopt support, this allows to get
the value set by earlier setsockopt.

Extends mptcp_put_int_option to handle u8 input/output by
adding required cast.

Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>