Split tap_ip_send() into IPv4 and IPv6 specific functions

The IPv4 and IPv6 paths in tap_ip_send() have very little in common, and
it turns out that every caller (statically) knows if it is using IPv4 or
IPv6.  So split into separate tap_ip4_send() and tap_ip6_send() functions.
Use a new tap_l2_hdr() function for the very small common part.

While we're there, make some minor cleanups:
  - We were double writing some fields in the IPv6 header, so that it
    temporary matched the pseudo-header for checksum calculation.  With
    recent checksum reworks, this isn't neccessary any more.
  - We don't use any IPv4 header options, so use some sizeof() constructs
    instead of some open coded values for header length.
  - The comment used to say that the flow label was for TCP over IPv6, but
    in fact the only thing we used it for was DHCPv6 over UDP traffic

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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