iscsi-target: Fix snprintf usage with MAX_PORTAL_LEN

This patch makes lio_target_call_addnptotpg() use sprintf() with
MAX_PORTAL_LEN + 1 to address the following smatch warning:

drivers/target/iscsi/iscsi_target_configfs.c +184 lio_target_call_addnptotpg(21)
        error: snprintf() chops off the last chars of 'name': 257 vs 256

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>