scsi_debug: change store from vmalloc to sgl

A long time ago this driver's store was allocated by kmalloc() or
alloc_pages(). When this was switched to vmalloc() the author
noticed slower ramdisk access times and more variability in repeated
tests. So try going back with sgl_alloc_order() to get uniformly
sized allocations in a sometimes large scatter gather _array_. That
array is the basis of keeping O(1) access to the store.

Using sgl_alloc_order() and friends requires CONFIG_SGL_ALLOC
so add a 'select' to the Kconfig file.

Remove kcalloc() in resp_verify() as sgl_s can now be compared
directly without forming an intermediate buffer. This is a
performance win for the SCSI VERIFY command implementation.

Make the SCSI COMPARE AND WRITE command yield the offset of the
first miscompared byte when the compare fails (as required by
T10).

This patch depends on: "[PATCH v4 0/4] scatterlist: add new
capabilities".

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>

drivers/scsi/Kconfig

@@ -1233,13 +1233,14 @@ config SCSI_DEBUG
 	tristate "SCSI debugging host and device simulator"
 	depends on SCSI
 	select CRC_T10DIF
+	select SGL_ALLOC
 	help
 	  This pseudo driver simulates one or more hosts (SCSI initiators),
 	  each with one or more targets, each with one or more logical units.
 	  Defaults to one of each, creating a small RAM disk device. Many
 	  parameters found in the /sys/bus/pseudo/drivers/scsi_debug
 	  directory can be tweaked at run time.
-	  See <http://sg.danny.cz/sg/sdebug26.html> for more information.
+	  See <http://sg.danny.cz/sg/scsi_debug.html> for more information.
 	  Mainly used for testing and best as a module. If unsure, say N.
 
 config SCSI_MESH