selftests/bpf: Test bpf_iter buffer access with negative offset

Commit afbf21dc ("bpf: Support readonly/readwrite buffers
in verifier") added readonly/readwrite buffer support which
is currently used by bpf_iter tracing programs. It has
a bug with incorrect parameter ordering which later fixed
by Commit f6dfbe31 ("bpf: Fix swapped arguments in calls
to check_buffer_access").

This patch added a test case with a negative offset access
which will trigger the error path.

Without Commit f6dfbe31, running the test case in the patch,
the error message looks like:
   R1_w=rdwr_buf(id=0,off=0,imm=0) R10=fp0
  ; value_sum += *(__u32 *)(value - 4);
  2: (61) r1 = *(u32 *)(r1 -4)
  R1 invalid (null) buffer access: off=-4, size=4

With the above commit, the error message looks like:
   R1_w=rdwr_buf(id=0,off=0,imm=0) R10=fp0
  ; value_sum += *(__u32 *)(value - 4);
  2: (61) r1 = *(u32 *)(r1 -4)
  R1 invalid rdwr buffer access: off=-4, size=4

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200728221801.1090406-1-yhs@fb.com