Merge branch 'rtnetlink-enable-IFLA_IF_NETNSID-for-RTM_DELLINK-RTM_SETINK'

Christian Brauner says:

====================
rtnetlink: enable IFLA_IF_NETNSID for RTM_{DEL,SET}LINK

Based on the previous discussion this enables passing a IFLA_IF_NETNSID
property along with RTM_SETLINK and RTM_DELLINK requests. The patch for
RTM_NEWLINK will be sent out in a separate patch since there are more
corner-cases to think about.

Changelog 2018-01-24:
* Preserve old behavior and report -ENODEV when either ifindex or ifname is
  provided and IFLA_GROUP is set. Spotted by Wolfgang Bumiller.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>