nbd: ignore non-nbd ioctl's

In testing we noticed that nbd would spew if you ran a fio job against
the raw device itself.  This is because fio calls a block device
specific ioctl, however the block layer will first pass this back to the
driver ioctl handler in case the driver wants to do something special.
Since the device was setup using netlink this caused us to spew every
time fio called this ioctl.  Since we don't have special handling, just
error out for any non-nbd specific ioctl's that come in.  This fixes the
spew.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>