mem: add validator callback

mem: add validator callback

This API will enable application to register for notifications
on page allocations that are about to happen, giving the application
a chance to allow or deny the allocation when total memory utilization
as a result would be above specified limit on specified socket.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Tested-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Tested-by: Hemant Agrawal <hemant.agrawal@nxp.com>
Tested-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>