selftests/bpf: BPF test_prog selftests for bpf_loop inlining

selftests/bpf: BPF test_prog selftests for bpf_loop inlining

Two new test BPF programs for test_prog selftests checking bpf_loop
behavior. Both are corner cases for bpf_loop inlinig transformation:
 - check that bpf_loop behaves correctly when callback function is not
   a compile time constant
 - check that local function variables are not affected by allocating
   additional stack storage for registers spilled by loop inlining

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/r/20220620235344.569325-6-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>