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>