mptcp: rename 'sk' to 'ssk' in mptcp_token_new_connect()

'ssk' should be more appropriate to be the name of the first argument
in mptcp_token_new_connect().

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>