for a read10 call, the application might have specified too few buffers
for the full I/O.
For example if the application tries to read just 512 bytes off a MMC device.
In this case we wopuld run out of buffers and fail with a SEGV.
Instead of failing like this, return NULL from the function to locate a suitable buffer and read the remaining data from the command into the callback buffer instead, just like when no read-buffer at all has been specified.
It is not real zero-copy since the data is still copied in the kernel,
but it avoids copying the data inside libiscsi as well as in the callback.
For SCSI tasks that will return data from the target, the application can now
specify application buffers for libiscsi to read the data directly into.
This is done by calling scsi_task_add_data_in_buffer(task, ...
These buffers need not be linear, you can specify different areas to read into
by calling this function several times.
See examples/iscsiclient.c for an example.
LBA is uint32_t for read/write10
Also store the lba/numblocks arguments in the task structure for read10/write10
This makes it much easier to implement a fully async "read lots/write lots"
applications.
