vxcan,veth: Forbid "type" for peer device

vxcan,veth: Forbid "type" for peer device

It is already given for original device we configure this
peer for.

Results from following command before/after change applied
are shown below:

  $ ip link add dev veth1a type veth peer name veth1b \
                           type veth peer name veth1c

Before:
-------

<no output, no netdevs created>

After:
------

Error: duplicate "type": "veth" is the second value.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>