mptcp: Add const qualifiers to functions in mptcp_output.c and mptcp_input.c

This patch adds const qualifiers to functions in mptcp_output.c and mptcp_input.c.

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