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>

selftests/bpf: Add bpf_loop test

Add test for bpf_loop testing a variety of cases:
various nr_loops, null callback ctx, invalid flags, nested callbacks.

Signed-off-by: Joanne Koong <joannekoong@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211130030622.4131246-3-joannekoong@fb.com