bpf: fix _htons occurences in test_progs

Dave reported that on sparc test_progs generates buggy swapped
eth->h_proto protocol comparisons:

  10: (15) if r3 == 0xdd86 goto pc+9
   R0=imm2,min_value=2,max_value=2 R1=pkt(id=0,off=0,r=14) R2=pkt_end R3=inv
   R4=pkt(id=0,off=14,r=14) R5=inv56 R10=fp

This is due to the unconditional ...

  #define htons __builtin_bswap16
  #define ntohs __builtin_bswap16

... in test_progs that causes this. Make use of asm/byteorder.h
and use __constant_htons() where possible and only perform the
bswap16 when on little endian in non-constant case.

Fixes: 6882804c ("selftests/bpf: add a test for overlapping packet range checks")
Fixes: 37821613 ("selftests/bpf: add l4 load balancer test based on sched_cls")
Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>