commit 8abc718d upstream.

In MC/S scenario, the conn->sess has been set NULL in
iscsi_login_non_zero_tsih_s1 when the second connection comes here,
then kernel panic.

The conn->sess will be assigned in iscsi_login_non_zero_tsih_s2. So
we should check whether it's NULL before calling.
Signed-off-by: Feng Li <lifeng1519@gmail.com>
Tested-by: Sumit Rai <sumit.rai@calsoftinc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>