Move the device lookup and checks to the ioctl handler under the RTNL and
change all name-based interfaces to take a struct net_device * instead.

This allows to use them from a netlink interface, which identifies devices
based on ifindex not name. It also avoids races between the ioctl interface
and the (upcoming) netlink interface since now all changes happen under the
RTNL.

As a nice side effect this greatly simplifies error handling in the helper
functions and fixes a number of incorrect error codes like -EINVAL for
device not found.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>