selftests/bpf: fix test_xdp_noinline on s390

test_xdp_noinline fails on s390 due to a handful of endianness issues.
Use ntohs for parsing eth_proto.
Replace bswaps with ntohs/htons.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>