mm: compaction: fix special case -1 order checks

Prevent division-by-zero panic when compaction was explicitely
requested through the procfs knob: /proc/sys/vm/compact_memory.

Backport from updtream commits 82478fb7 and 3957c776.

Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
mm: compaction: add compaction sysctl

Add a new sysctl to enable/disable memory compaction and
the default is enabled.

Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
Merge pull request #58 from shazj99/compaction

Memory compaction enhancement