iscsi-target: Re-add chunk from backport of upstream 79d59d08082d to 3.10

Commit d5c55fa3, the backport of upstream 79d59d08082d
("iscsi-target: Fix wrong buffer / buffer overrun in
iscsi_change_param_value()") left out applying one chunk of the fix in
iscsi_login_non_zero_tsih_s2().  Add the missing chunk.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>