linux-user: SOCK_PACKET uses network endian to encode protocol in socket()

linux-user: SOCK_PACKET uses network endian to encode protocol in socket()

in PACKET(7) :

  packet_socket = socket(AF_PACKET, int socket_type, int protocol);
[...]
                                   protocol is the  IEEE  802.3 protocol
  number in network order.  See the <linux/if_ether.h> include file for a
  list of allowed protocols.  When protocol is  set  to htons(ETH_P_ALL)
  then all protocols are received.  All incoming packets of that protocol
  type will be passed to the packet socket before they are passed to the
  protocols implemented in the kernel.
[...]
Compatibility

  In Linux 2.0, the only way to  get  a  packet  socket  was  by  calling
  socket(AF_INET,  SOCK_PACKET,  protocol).

We need to tswap16() the protocol because on big-endian, the ABI is
waiting for, for instance for ETH_P_ALL, 0x0003 (big endian ==
network order), whereas on little-endian it is waiting for 0x0300.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>