target/iscsi: Fix mutual CHAP auth on big-endian arches

See https://bugzilla.redhat.com/show_bug.cgi?id=916290



Used a temp var since we take its address in sg_init_one.

Signed-off-by: Andy Grover <agrover@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>