target: REPORT LUNS should return LUN 0 even for dynamic ACLs

If an initiator doesn't have any real LUNs assigned, we should report
LUN 0 and a LUN list length of 1.  Some versions of Solaris at least
go beserk if we report a LUN list length of 0.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: <stable@vger.kernel.org> # v3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>