scsi: iscsi_tcp: Check that sock is valid before iscsi_set_param()

scsi: iscsi_tcp: Check that sock is valid before iscsi_set_param()

The validity of sock should be checked before assignment to avoid incorrect
values. Commit 57569c37f0ad ("scsi: iscsi: iscsi_tcp: Fix null-ptr-deref
while calling getpeername()") introduced this change which may lead to
inconsistent values of tcp_sw_conn->sendpage and conn->datadgst_en.

Fix the issue by moving the position of the assignment.

Fixes: 57569c37f0ad ("scsi: iscsi: iscsi_tcp: Fix null-ptr-deref while calling getpeername()")
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>
Link: https://lore.kernel.org/r/20230329071739.2175268-1-zhongjinghua@huaweicloud.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>