mptcp: Fix Issue #37: Explicitly cast srtt to 64-bit

We may have an integer overflow when multiplying the srtt's. C does not
do an automatic cast of 32-bit integers to 64-bit integers. So, we have
to make it explicit to avoid the integer overflow.

Fixes: Bug #37 (https://github.com/multipath-tcp/mptcp/issues/37)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>