mptcp: zero token hash at creation time.

Otherwise the 'chain_len' filed will carry random values,
some token creation calls will fail due to excessive chain
length, causing unexpected fallback to TCP.

Fixes: 2c5ebd00 ("mptcp: refactor token container")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Tested-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>