misannotation in pppol2tp

Address of auto variable is not a userland pointer.  A good thing, too,
since if pppol2tp_tunnel_getsockopt() would _really_ get a userland pointer
as argument, it would be an instant roothole...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>