iscsi-target: Fix residual count hanlding + remove iscsi_cmd->residual_count

iscsi-target: Fix residual count hanlding + remove iscsi_cmd->residual_count

This patch fixes iscsi-target handling of underflow where residual data is
causing an OOPs by using the incorrect iscsi_cmd_t->data_length initially
assigned in iscsit_allocate_se_cmd().  It resets iscsi_cmd_t->data_length
from se_cmd_t->data_length after transport_generic_allocate_tasks()
has been invoked in iscsit_handle_scsi_cmd() RX context, and converts
iscsi_cmd->residual_count usage to access iscsi_cmd->se_cmd.residual_count
to get the proper residual count set by target-core.

Reported-by: <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Andy Grover <[email protected]>
Cc: [email protected]
Signed-off-by: Nicholas Bellinger <[email protected]>