selftests/bpf: Fix misaligned memory access in queue_stack_map test

Copy over iphdr into a local variable before accessing its fields.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124002325.1737739-11-andrii@kernel.org