Don't accidentially use user space ntohs function.

Don't accidentially use user space ntohs function.

The definition of ntohs comes from user space include headers
netinet/in.h which could define it as function instead of macro.
So use builtin kernel be16_to_cpu instead.

* tapset/aux_syscalls.stp (_struct_sockaddr_u): Define and use _stp_ntohs
  as be16_to_cpu.