bpf, tests: Add tests for atomic operations

Tests for each atomic arithmetic operation and BPF_XCHG, derived from
old BPF_XADD tests. The tests include BPF_W/DW and BPF_FETCH variants.
Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210809091829.810076-13-johan.almbladh@anyfinetworks.com