perf bench: Fix order of arguments to memcpy_alloc_mem

perf bench: Fix order of arguments to memcpy_alloc_mem

This was causing the destination instead of the source to be filled.  As
a result, the source was typically all mapped to one zero page, and
hence very cacheable.

Signed-off-by: Bruce Merry <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/20150115092022.GA11292@kryton
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>