node_memory: Add new parameter field to tune the new sysfs knob

node_memory: Add new parameter field to tune the new sysfs knob

Upstream kernel introduced new sysfs knob "merge_across_nodes" to
specify if pages from different numa nodes can be merged. When set
to 0, only pages which physically reside in the memory area of
same NUMA node can be merged. When set to 1, pages from all nodes
can be merged.

This patch supports the tuning by adding new param field
"shm_merge_across_nodes".