[PPP_MPPE]: Don't put InterimKey on the stack

ppp_mppe puts a crypto key on the kernel stack, then passes the
address of that into the crypto layer.  That doesn't work because the
crypto layer needs to be able to do virt_to_*() on the address which
does not universally work for the kernel stack on all platforms.

Adrian Bunk:
Backported to 2.6.16.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Adrian Bunk <bunk@kernel.org>