mem: prepare memseg lists for multiprocess sync

mem: prepare memseg lists for multiprocess sync

In preparation for implementing multiprocess support, we are adding
a version number to memseg lists. We will not need any locks, because
memory hotplug will have a global lock (so any time memory map and
thus version number might change, we will already be holding a lock).

There are two ways of implementing multiprocess support for memory
hotplug: either all information about mapped memory is shared
between processes, and secondary processes simply attempt to
map/unmap memory based on requests from the primary, or secondary
processes store their own maps and only check if they are in sync
with the primary process' maps.

This implementation will opt for the latter option: primary process
shared mappings will be authoritative, and each secondary process
will use its own interal view of mapped memory, and will attempt
to synchronize on these mappings using versioning.

Under this model, only primary process will decide which pages get
mapped, and secondary processes will only copy primary's page
maps and get notified of the changes via IPC mechanism (coming
in later commits).

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>