macvlan: better mode validation
macvlan: better mode validation

macvlan passthrough mode is special: it's not possible to switch to or
from it through a netlink command.

But if you try, the command will succeed, which is
confusing.

Validate input and return error to user.

Cc:  Sridhar Samudrala <532f110e089ab7f9606f1ba1fe4648a4a4d5dcc2@us.ibm.com>
Cc: "David S. Miller" <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>
Signed-off-by: Michael S. Tsirkin <255103e50249e3d658441816e0597170ebfc16ef@redhat.com>
Signed-off-by: David S. Miller <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs