obsolete the old api.
After discussions, It is probably not the right thing to do.
Lets leave the old api as is. Most simpler applications will
continue to pass a single linear buffer to the iscsi_<send-data-out-to-device>_task() for convenience so it would be wrong to force them all to
migrate to amore complex iovector api.
Convert any linear buffers, if provided, to a one element data-out iovector
if a buffer was provided.
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