target: prevent NULL pointer dereference in target_report_luns

transport_kmap_data_sg can return NULL.  I never saw this trigger, but
returning -ENOMEM seems better than a crash.  Also removes a pointless
case while at it.

Signed-off-by: Joern Engel <joern@logfs.org>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>