mptcp: Fix returned type of some functions

This patch fix returned type of before64 and mptcp_sequence from int to
bool.

Signed-off-by: Kenjiro Nakayama <nakayamakenjiro@gmail.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
mptcp: Bool data type should explicitly return false

Bool data type should explicitly return false not 0.

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