SCSI sg on 20190709

This topic branch covers a fundamental change in how our sg lists are
allocated to make mq more efficient by reducing the size of the
preallocated sg list.  This necessitates a large number of driver
changes because the previous guarantee that if a driver specified
SG_ALL as the size of its scatter list, it would get a non-chained
list and didn't need to bother with scatterlist iterators is now
broken and every driver *must* use scatterlist iterators.

This was broken out as a separate topic because we need to convert all
the drivers before pulling the trigger and unconverted drivers kept
being found, necessitating a rebase.

Signed-off-by: James E.J. Bottomley <jejb@linux.ibm.com>
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