mem: check for memfd support in segment fd API

mem: check for memfd support in segment fd API

If memfd support was not compiled, or hugepage memfd support
is not available at runtime, the API will now return proper
error code, indicating that this API is unsupported. This
changes the API, so document the changes.

Fixes: 41dbdb68723b ("mem: add external API to retrieve page fd")
Fixes: 3a44687139eb ("mem: allow querying offset into segment fd")
Cc: stable@dpdk.org
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>