net/virtio-user: drop attribute unused for memory callback

net/virtio-user: drop attribute unused for memory callback

The "addr" param has been used since the event callbacks are enabled
for external memory. So the "__rte_unused" should be dropped.

Besides, slightly refine the coding style by consistently assuming
tabs are 8 characters.

Fixes: f32c7c9de961 ("malloc: enable event callbacks for external memory")
Cc: stable@dpdk.org
Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>