iscsi-target: Fix tfc_tpg_auth_cit configfs length overflow

This patch fixes another potential buffer overflow while processing
iscsi_node_auth input for configfs attributes in v3.11 for-next
TPG tfc_tpg_auth_cit context.

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