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>

include/linux/if_pppol2tp.h

@@ -39,7 +39,7 @@ struct pppol2tp_addr {
  * bits. So we need a different sockaddr structure.
  */
 struct pppol2tpv3_addr {
-	pid_t	pid;			/* pid that owns the fd.
+	__kernel_pid_t	pid;		/* pid that owns the fd.
 					 * 0 => current */
 	int	fd;			/* FD of UDP or IP socket to use */