x86/mm: Avoid duplicated pxm_to_node() calls

In slit init code, too many pxm_to_node() function calls are done.

We can store from_node/to_node instead of keep calling
pxm_to_node().

  - Before this patch: pxm_to_node() is called n*(1+n*3) times.
  - After  this patch: pxm_to_node() is called n*(1+n) times.

for  8 sockets, it will be   72 instead of  200.
for 32 sockets, it will be 1056 instead of 3104.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Toshi Kani <toshi.kani@hp.com>
Cc: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/1390770102-4007-1-git-send-email-yinghai@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>