pptp: Fix fib lookup calls.

PPTP uses pppox sockets (struct pppox_sock). These sockets don't embed
an inet_sock structure, so it's invalid to call inet_sk() on them.

Therefore, the ip_route_output_ports() call in pptp_connect() has two
problems:

  * The tos variable is set with RT_CONN_FLAGS(sk), which calls
    inet_sk() on the pppox socket.

  * ip_route_output_ports() tries to retrieve routing flags using
    inet_sk_flowi_flags(), which is also going to call inet_sk() on the
    pppox socket.

While PPTP doesn't use inet sockets, it's actually really layered on
top of IP and therefore needs a proper way to do fib lookups. So let's
define pptp_route_output() to get a struct rtable from a pptp socket.
Let's also replace the ip_route_output_ports() call of pptp_xmit() for
consistency.

In practice, this means that:

  * pptp_connect() sets ->flowi4_tos and ->flowi4_flags to zero instead
    of using bits of unrelated struct pppox_sock fields.

  * pptp_xmit() now respects ->sk_mark and ->sk_uid.

  * pptp_xmit() now calls the security_sk_classify_flow() security
    hook, thus allowing to set ->flowic_secid.

  * pptp_xmit() now passes the pppox socket to xfrm_lookup_route().

Found by code inspection.

Fixes: 00959ade ("PPTP: PPP over IPv4 (Point-to-Point Tunneling Protocol)")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>