mptcp: move the declarations of ssk and subflow

Move the declarations of ssk and subflow in MP_FAIL and MP_PRIO to the
beginning of the function mptcp_write_options().

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>