mem: add API to lock/unlock memory hotplug

mem: add API to lock/unlock memory hotplug

Currently, the memory hotplug is locked automatically by all
memory-related _walk() functions, but sometimes locking the
memory subsystem outside of them is needed. There is no
public API to do that, so it creates a dependency on shared
memory config to be public. Fix this by introducing a new
API to lock/unlock the memory hotplug subsystem.

Create a new common file for all things mem config, and a
new API namespace rte_mcfg_*, and search-and-replace all
usages of the locks with the new API.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: David Marchand <david.marchand@redhat.com>