target/user: Fix comments to not refer to data ring

We no longer use a ringbuffer for the data area, so this might cause
confusion. Just call it the data area.

Signed-off-by: Andy Grover <agrover@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>