bpf: fix up selftests after backports were fixed

After the backport of the changes to fix CVE 2019-7308, the
selftests also need to be fixed up, as was done originally
in mainline 80c9b2fa ("bpf: add various test cases to selftests").

This is a backport of upstream commit 80c9b2fa

 ("bpf: add various test
cases to selftests") adapted to 4.19 in order to fix the
selftests that began to fail after CVE-2019-7308 fixes.

Suggested-by: Frank van der Linden <fllinden@amazon.com>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>