iscsi-target: Fix tfc_tpg_nacl_auth_cit configfs length overflow

This patch fixes a potential buffer overflow while processing
iscsi_node_auth input for configfs attributes within NodeACL
tfc_tpg_nacl_auth_cit context.

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