bpf: add test cases for direct packet access

Add couple of test cases for direct write and the negative size issue, and
also adjust the direct packet access test4 since it asserts that writes are
not possible, but since we've just added support for writes, we need to
invert the verdict to ACCEPT, of course. Summary: 133 PASSED, 0 FAILED.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>