libnetlink: Use ifinfomsg instead of rtgenmsg in rtnl_wilddump_req_filter

This change corrects a kernel incompatibility that was resulting in the
ext_filter_mask not being correctly discovered by the kernel as it is buried
somewhere in the ifinfomsg.

Reported-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: David S. Miller <davem@davemloft.net>