PPPOE: fix memory leak (local DoS) (CVE-2007-2525)

This patch fixes a memory leak when a PPPoE socket is release()d after
it has been connect()ed, but before the PPPIOCGCHAN ioctl ever has been
called on it.

This is somewhat of a security problem, too, since PPPoE sockets can be
created by any user, so any user can easily allocate all the machine's
RAM to non-swappable address space and thus DoS the system.

Is there any specific reason for PPPoE sockets being available to any
unprivileged process, BTW? After all, you need a packet socket for the
discovery stage anyway, so it's unlikely that any unprivileged process
will ever need to create a PPPoE socket, no? Allocating all session IDs
for a known AC is a kind of DoS, too, after all - with Juniper ERXes,
this is really easy, actually, since they don't ever assign session ids
above 8000 ...

Signed-off-by: Florian Zumbiehl <florz@florz.de>
Signed-off-by: Adrian Bunk <bunk@kernel.org>