openvswitch: switch from WARN to pr_warn

As noted by Paolo Abeni, pr_warn doesn't generate any splat and can still
preserve the warning to the user that feature downgrade occurred.  We
likely cannot introduce other kinds of checks / enforcement here because
syzbot can generate different genl versions to the datapath.

Reported-by:  <syzbot+31cde0bef4bbf8ba2d86@syzkaller.appspotmail.com>
Fixes: 44da5ae5

 ("openvswitch: Drop user features if old user space attempted to create datapath")
Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Aaron Conole <aconole@redhat.com>
Acked-by: Ilya Maximets <i.maximets@ovn.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>