net: filter: fix nlattr and nlattr_nest BPF tests

- 'struct nlattr' must be 2 byte aligned
- provide big-endian input data for nlattr/nlattr_nest tests

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>