bpf/tests: Add zero-extension checks in BPF_ATOMIC tests

This patch updates the existing tests of BPF_ATOMIC operations to verify
that a 32-bit register operand is properly zero-extended. In particular,
it checks the operation on archs that require 32-bit operands to be
properly zero-/sign-extended or the result is undefined, e.g. MIPS64.

Signed-off-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211001130348.3670534-3-johan.almbladh@anyfinetworks.com