ipvlan: don't check for valid @mode argument in rtnl_link_ipvlan_set_mode()

ipvlan: don't check for valid @mode argument in rtnl_link_ipvlan_set_mode()

In the future kernel might support more modes. Don't be so
strict in rtnl_link_ipvlan_set_mode() and accept any uint16
mode.
This way when adding new modes, rtnl_link_ipvlan_set_mode() does not
need to be changed.

If the user passes an invalid value and sends a message to the kernel,
it will be rejected there.

http://lists.infradead.org/pipermail/libnl/2015-June/001902.html

Fixes: 7de5be85bf9aa3eb9f022e4813226135e89adec2
Signed-off-by: Thomas Haller <thaller@redhat.com>