mm: only define hashdist variable when needed

mm: only define hashdist variable when needed

For !CONFIG_NUMA, hashdist will always be 0, since it's setter is
otherwise compiled out.  So we can save 4 bytes of data and some .text
(although mostly in __init functions) by only defining it for
CONFIG_NUMA.

Signed-off-by: Rasmus Villemoes <[email protected]>
Acked-by: David Rientjes <[email protected]>
Reviewed-by: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>