benchtests: Move iterator declaration into loop header

This is a minor style change to move the definition of I to its usage
scope instead of at the top of the function.  This is consistent with
glibc style guidelines and more importantly it was getting in the way
of my testing.

	* benchtests/bench-memcpy-walk.c (do_test): Move declaration
	of I into loop header.
	* benchtests/bench-memmove-walk.c (do_test): Likewise.