iscsi-target: Fix incorrect buffer usage in non-leading connection login

iscsi-target: Fix incorrect buffer usage in non-leading connection login

This patch fixes a bug in non-leading connection login processing where
iscsi_login_non_zero_tsih_s2() was incorrectly using the login request
header as a temporary buffer for setting TargetPortalGroupTag.

This bug was manifesting itself as login failures with non-leading login
requests in multiple connection per session (MC/S) operation, and has
been present since the initial v3.1 merge of iscsi-target code.

However, the bug has only been able to trigger since the v3.10 changes
in commit baa4d64b1 where included to support iser-target.

Reported-by: Santosh Kulkarni <santosh.kulkarni@calsoftinc.com>
Cc: stable@vger.kernel.org # 3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>