target: simplify target_fabric_make_lun error path

Coverity complained that lun_cg has been dereferenced in all paths
leading to NULL check.  It didn't mention that only a single path could
lead there and the code can be simplified even further.

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