malloc: add function to check if socket is external

malloc: add function to check if socket is external

An API is needed to check whether a particular socket ID belongs
to an internal or external heap. Prime user of this would be
mempool allocator, because normal assumptions of IOVA
contiguousness in IOVA as VA mode do not hold in case of
externally allocated memory.

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