benchtests: Fix walking sizes and directions for *-walk benchmarks

Make the walking benchmarks walk only backwards since copying both
ways is biased in favour of implementations that use non-temporal
stores for larger sizes; falkor is one of them.  This also fixes up
bugs in computation of the result which ended up multiplying the
length with the timing result unnecessarily.

	* benchtests/bench-memcpy-walk.c (do_one_test): Copy only
	backwards.  Fix timing computation.
	* benchtests/bench-memmove-walk.c (do_one_test): Likewise.
	* benchtests/bench-memset-walk.c (do_one_test): Walk backwards
	on memset by N at a time.  Fix timing computation.