selftests/bpf: Test subprog jit when toggle bpf_jit_harden repeatedly

When bpf_jit_harden is toggled between 0 and 2, subprog jit may fail
due to inconsistent twice read values of bpf_jit_harden during jit. So
add a test to ensure the problem is fixed.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220309123321.2400262-5-houtao1@huawei.com