When DPDK is being compiled in C++ project using g++ then
'invalid conversion from' error appears. Added explicit
typecast on function return to get rid of the error.
Fixes: 6cf14ce4ce6c ("mempool: silence warning on pointer arithmetic")
Signed-off-by: Sergey Balabanov <balabanovsv@ecotelecom.ru>
Acked-by: Olivier Matz <olivier.matz@6wind.com>