mptcp: Add const qualifiers to functions in mptcp_ctrl.c, mptcp_ipv4.c and mptcp_ipv6.c

This patch adds const qualifiers to functions in mptcp_ctrl.c,
mptcp_ipv4.c and mptcp_ipv6.c

Signed-off-by: Kenjiro Nakayama <nakayamakenjiro@gmail.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>