nx-match: Refactor nxm_put_ip() to handle all IPv4 and IPv6 fields.

Until now, some fields have been handled in the caller, and the caller has
been responsible for distinguishing ICMPv4 from ICMPv6.  This
implementation seems to make the code a little easier to understand.

Signed-off-by: Ben Pfaff <blp@nicira.com>
work on ext-109