bpf: Fix selftests are changes for CVE 2019-7308

The changes to fix the CVE 2019-7308 make the bpf verifier stricter
with respect to operations that were allowed earlier in unprivileged
mode. Fixup the test cases so that the error messages now correctly
reflect pointer arithmetic going out of range for tests.

Signed-off-by: Balbir Singh <sblbir@amzn.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>