These just want to return a pointer instead of a value, but are otherwise
the same.

ISCSI_TPG_LUN macro was unused.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>