iscsi-target: Fix up a few assignments

A statement such as
  struct iscsi_node_attrib *na = na = iscsit_tpg_get_node_attrib(sess);
has undefined behaviour since there are two assignments to 'na', strictly
speaking (the order in which side-effects from the assignments take place
is undefined since there's no intervening sequence point), and it looks
unintentional in any case.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>