compat_ioctl: add compat handler for PPPIOCGL2TPSTATS

fixes following error seen on x86_64 kernel:
ioctl32(openl2tpd:7480): Unknown cmd fd(14) cmd(80487436){t:'t';sz:72} arg(ffa7e6c0) on socket:[105094]

The argument (struct pppol2tp_ioc_stats) uses "aligned_u64" and thus doesn't need
fixups.

Cc: James Chapman <jchapman@katalix.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>