mm: migrate: support multiple target nodes demotion

We have some machines with multiple memory types like below, which have
one fast (DRAM) memory node and two slow (persistent memory) memory
nodes.  According to current node demotion policy, if node 0 fills up,
its memory should be migrated to node 1, when node 1 fills up, its
memory will be migrated to node 2: node 0 -> node 1 -> node 2 ->stop.

But this is not efficient and suitbale memory migration route for our
machine with multiple slow memory nodes.  Since the distance between
node 0 to node 1 and node 0 to node 2 is equal, and memory migration
between slow memory nodes will increase persistent memory bandwidth
greatly, which will hurt the whole system's performance.

Thus for this case, we can treat the slow memory node 1 and node 2 as a
whole slow memory region, and we should migrate memory from node 0 to
node 1 and node 2 if node 0 fills up.

This patch changes the node_demotion data structure to support multiple
target nodes, and establishes the migration path to support multiple
target nodes with validating if the node distance is the best or not.

  available: 3 nodes (0-2)
  node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
  node 0 size: 62153 MB
  node 0 free: 55135 MB
  node 1 cpus:
  node 1 size: 127007 MB
  node 1 free: 126930 MB
  node 2 cpus:
  node 2 size: 126968 MB
  node 2 free: 126878 MB
  node distances:
  node   0   1   2
    0:  10  20  20
    1:  20  10  20
    2:  20  20  10

Link: https://lkml.kernel.org/r/00728da107789bb4ed9e0d28b1d08fd8056af2ef.1636697263.git.baolin.wang@linux.alibaba.com
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: zhongjiang-ali <zhongjiang-ali@linux.alibaba.com>
Cc: Xunlei Pang <xlpang@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>