This series add support for rtl8_4t tag. It is a variant of rtl8_4 tag,
with identical values but placed at the end of the packet. 

It forces checksum in software before adding the tag as those extra
bytes at the end of the packet would be summed together with the rest of
the payload. When the switch removes the tag before sending the packet
to the network, that checksum will not match.

It might be useful to diagnose or avoid checksum offload issues. With an
ethertype tag like rtl8_4, the cpu port ethernet driver must work with
cksum_start and chksum_offset in order to correctly calculate checksums.
If not, the checksum field will be broken (it will contain the fake ip
header sum).  In those cases, using 'rtl8_4t' might be an alternative
way to avoid checksum offload, either using runtime or device-tree
property.

Regards,

Luiz

v1-v2)
- Remove mention to tail tagger, use trailing tagger.
- use void* instead of char* for pointing to tag beginning
- use memcpy to avoid problems with unaligned tags
- calculate checksum if it still pending
- keep in-use tag protocol in memory instead of reading from switch
  register