[PPPOL2TP]: Fix endianness annotations.

{s,d}_{session,tunnel} in pppol2tp_addr are actually host-endian
everywhere.  We might switch them to net-endian, of course, but
that structure is exposed to userland via getname...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>