iscsi-target: Fix tfc_tpg_nacl_auth_cit configfs length overflow

commit 0fbfc46fb0b2f543a8b539e94c6c293ebc0b05a6 upstream.

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>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>