create_link_id() could be more descriptive when link already exists

@leftwo reported seeing the following failure when trying to create a guest instance with Omicron:

"error_message_external":"Internal Server Error","error_message_internal":"Error managing instances: Instance error: Failure interacting with the OPTE ioctl(2) interface: netadm failed dlmgmtd: link id creation failed: 17","response_code":"500"

The Failure interacting with the OPTE ioctl(2) interface: part of the message comes from sled-agent, which presents this message anytime a call to opte_ioctl::OpteHdl fails. However, technically, this failure is on the userspace side, before calling the OPTE ioctl. Specifically the failure happens when requesting a new link ID from dlmgmtd.

        let linkid = link::create_link_id(
            name,
            libnet::LinkClass::Xde,
            libnet::LinkFlags::Active,
        )?;

The link id creation failed: 17 comes from this library, and it indicates the call to create the new link id failed with EEXIST (errno = 17).

    if response.linkid == 0 || response.err != 0 {
        return Err(Error::Dlmgmtd(format!(
            "link id creation failed: {}",
            response.err
        )));
    }

We could improve this specific error message by matching against response.err. If it's equal to EEXIST, we could return a libnet::Error::AlreadyExists(String) to make it more obvious to those of us (like me) who fail to memorize errno.h.