ping: populate icmp_id field for "simple" ping too

ping: populate icmp_id field for "simple" ping too

The ICMP RFC says that identifier and sequence number may be zero.
Having them zero for a Echo message, along with a data of zero's
as well will result in a Echo reply message with only zero's.

Some NAT implementations seem to get the checksum wrong on these
packages. Setting a checksum of 0x0 instead of 0xffff.

Through NAT:
  Internet Control Message Protocol
      Type: 0 (Echo (ping) reply)
      Code: 0
      Checksum: 0x0000 [incorrect, should be 0xffff]
      Identifier (BE): 0 (0x0000)
      Identifier (LE): 0 (0x0000)
      Sequence number (BE): 0 (0x0000)
      Sequence number (LE): 0 (0x0000)
      Data (56 bytes)
          Data: 000000000000000000000000000000000000000000000000...
          [Length: 56]

Without NAT:
  Internet Control Message Protocol
      Type: 0 (Echo (ping) reply)
      Code: 0
      Checksum: 0xffff [correct]
      Identifier (BE): 0 (0x0000)
      Identifier (LE): 0 (0x0000)
      Sequence number (BE): 0 (0x0000)
      Sequence number (LE): 0 (0x0000)
      [Request frame: 189]
      [Response time: 0.024 ms]
      Data (56 bytes)
          Data: 000000000000000000000000000000000000000000000000...
          [Length: 56]

And this in turn will make some hardware MAC checksum offloading
engines drop the packet.

(This was seen with a Synopsis MAC, the same one used in for instance the
stmmac Ethernet driver in the linux kernel.)

This change can be seen as a workaround for bugs in other layers.
But just setting an identifier for the Echo message packet will
avoid prodding the hornets nest.

function                                             old     new   delta
common_ping_main                                     424     500     +76

Signed-off-by: Jonas Danielsson <jonasdn@axis.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>