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: <lists@internyc.net> Cc: Christoph Hellwig <hch@lst.de> Cc: Andy Grover <agrover@redhat.com> Cc: stable@kernel.org Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>