mptcp: init mptcp request socket earlier

The mptcp subflow route_req() callback performs the subflow
req initialization after the route_req() check. If the latter
fails, mptcp-specific bits of the current request sockets
are left uninitialized.

The above causes bad things at req socket disposal time, when
the mptcp resources are cleared.

This change addresses the issue by splitting subflow_init_req()
into the actual initialization and the mptcp-specific checks.
The initialization is moved before any possibly failing check.

Reported-by: Christoph Paasch <cpaasch@apple.com>
Fixes: 7ea851d1

 ("tcp: merge 'init_req' and 'route_req' functions")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>