scsi: target: tcmu: Optimize scatter_data_area()

scatter_data_area() has two purposes:

 1) Create the iovs for the data area buffer of a SCSI cmd.

 2) If there is data in DMA_TO_DEVICE direction, copy
    the data from sg_list to data area buffer.

Both are done in a common loop.

In case of DMA_FROM_DEVICE data transfer, scatter_data_area() is called
with parameter copy_data = false. But this flag is just used to skip
memcpy() for data, while radix_tree_lookup still is called for every dbi of
the area area buffer, and kmap and kunmap are called for every page from
sg_list and data_area as well as flush_dcache_page() for the data area
pages.  Since the only thing to do with copy_data = false would be to set
up the iovs, this is a noticeable overhead.  Rework the iov creation in the
main loop of scatter_data_area() providing the new function
new_block_to_iov().  Based on this, create the short new function
tcmu_setup_iovs() that only writes the iovs with no overhead.  This new
function is now called instead of scatter_data_area() for bidi buffers and
for data buffers in those cases where memcpy() would have been skipped.

Link: https://lore.kernel.org/r/20200910155041.17654-4-bstroesser@ts.fujitsu.com
Acked-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>