x86-64, NUMA: Seperate out numa_alloc_distance() from numa_set_distance()

Alloc code is much bigger the distance setting.  Separate it out into
numa_alloc_distance() for readability.

-v2: Let alloc_numa_distance to return -ENOMEM on failing path,
     requested by tj.

-tj: Description update.  Minor tweaks including function name,
     location and return value check.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>