iscsi-target: return -EINVAL on oversized configfs parameter

The iSCSI CHAP auth parameters are already copied with respect for
the destination buffer size.  Return -EINVAL instead of silently
truncating the input.

Signed-off-by: Eric Seppanen <eric@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_configfs.c

@@ -474,7 +474,8 @@ static ssize_t __iscsi_##prefix##_store_##name(				\
 									\
 	if (!capable(CAP_SYS_ADMIN))					\
 		return -EPERM;						\
-									\
+	if (count >= sizeof(auth->name))				\
+		return -EINVAL;						\
 	snprintf(auth->name, sizeof(auth->name), "%s", page);		\
 	if (!strncmp("NULL", auth->name, 4))				\
 		auth->naf_flags &= ~flags;				\