uapi: fix linux/if_pppol2tp.h userspace compilation errors

Because of <linux/libc-compat.h> interface limitations, <netinet/in.h>
provided by libc cannot be included after <linux/in.h>, therefore any
header that includes <netinet/in.h> cannot be included after <linux/in.h>.

Change uapi/linux/l2tp.h, the last uapi header that includes
<netinet/in.h>, to include <linux/in.h> and <linux/in6.h> instead of
<netinet/in.h> and use __SOCK_SIZE__ instead of sizeof(struct sockaddr)
the same way as uapi/linux/in.h does, to fix linux/if_pppol2tp.h userspace
compilation errors like this:

In file included from /usr/include/linux/l2tp.h:12:0,
                 from /usr/include/linux/if_pppol2tp.h:21,
/usr/include/netinet/in.h:31:8: error: redefinition of 'struct in_addr'

Fixes: 47c3e778 ("net: l2tp: deprecate PPPOL2TP_MSG_* in favour of L2TP_MSG_*")
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: David S. Miller <davem@davemloft.net>