selftests/bpf: Simplify the calculation of variables

Fix the following coccicheck warnings:

./tools/testing/selftests/bpf/xdpxceiver.c:954:28-30: WARNING !A || A &&
B is equivalent to !A || B.

./tools/testing/selftests/bpf/xdpxceiver.c:932:28-30: WARNING !A || A &&
B is equivalent to !A || B.

./tools/testing/selftests/bpf/xdpxceiver.c:909:28-30: WARNING !A || A &&
B is equivalent to !A || B.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1612860398-102839-1-git-send-email-jiapeng.chong@linux.alibaba.com