Allocate and free (NR_SG - 1) parameter RAM slots, and make max_hw_segs
reflect that.  This is a NOP until NR_SG is changed, at which point
those slots *need* to be used.

Unmapping scatterlists must use the original scatterlist length, not
the length after mapping.  (But they won't currently differ.)
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>