test_bpf: Add backward jump test case

Currently the testsuite does not have a test case with a backward jump.
The s390x JIT (kernel 4.0) had a bug in that area.
So add one new test case for this now.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>