mptcp: Do not overwrite mptcp_seed in mptcp_v6_get_key

mptcp_seed is set in secret[5]. The loop right after it, starts again at
5 to set mptcp_secret there. Thus, mptcp_seed is always overwritten and
if we have a token-collision we do not have the necessary entropy to
actually generate a different key.

Reported-by: John Ronan <jpronans@gmail.com>
Fixes: 560443b12b731 (Don't use get_random_bytes)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>