headers, pppol2tp: Use __kernel_pid_t in <linux/pppol2tp.h>

<linux/types.h> defines __kernel_pid_t for userland; pid_t is
defined elsewhere (and potentially differently).

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>