mem: provide thread-unsafe memseg walk variant

mem: provide thread-unsafe memseg walk variant

Sometimes, user code needs to walk memseg list while being inside
a memory-related callback. Rather than making everyone copy around
the same iteration code and depending on DPDK internals, provide an
official way to do memseg_walk() inside callbacks.

Also, remove existing reimplementation from sPAPR VFIO code and use
the new API instead.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>