Some targets may return PQ=1 and PDT=0x1f to indicate no LUN is mapped
(Netapp targets do this).  This seems like a valid way to indicate no
LUN mapped according to SPC-3.

However, the current scsi_probe_and_add_lun() code adds a scsi_device
for targets that return PQ=1 and PDT=0x1f.  This causes LUNs of type
"UNKNOWN" to show up in /proc/scsi/scsi when no LUNs are mapped.
In addition, subsequent rescans fail to recognize LUNs that may be
added on the target, unless preceded by a write to the delete attribute
of the "UNKNOWN" LUN.

This patch addresses this problem by skipping over the scsi_add_lun()
when PQ=1,PDT=0x1f is encountered, and just returns
SCSI_SCAN_TARGET_PRESENT.

If there are objections to this patch, I can add a BLIST flag and entry
for Netapp targets but would like to avoid that if possible, since it
seems like the current code might be closer to SPC-3 with this patch.

Signed-off-by: Dave Wysochanski <davidw@netapp.com>