bpf: fix and add test cases for ARG_CONST_SIZE_OR_ZERO semantics change

Fix a few test cases to allow non-NULL map/packet/stack pointer
with size = 0. Change a few tests using bpf_probe_read to use
bpf_probe_write_user so ARG_CONST_SIZE arg can still be properly
tested. One existing test case already covers size = 0 with non-NULL
packet pointer, so add additional tests so all cases of
size = 0 and 0 <= size <= legal_upper_bound with non-NULL
map/packet/stack pointer are covered.

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