When the name_assign_type attribute was introduced (commit
685343fc, "net: add name_assign_type netdev attribute"), the
loopback device was explicitly mentioned as one which would make use
of NET_NAME_PREDICTABLE:

    The name_assign_type attribute gives hints where the interface name of a
    given net-device comes from. These values are currently defined:
...
      NET_NAME_PREDICTABLE:
        The ifname has been assigned by the kernel in a predictable way
        that is guaranteed to avoid reuse and always be the same for a
        given device. Examples include statically created devices like
        the loopback device [...]

Switch to that so that reading /sys/class/net/lo/name_assign_type
produces something sensible instead of returning -EINVAL.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>