Clarify that the mempool private initializer and object initializer used
for packet pools require that the mempool private size is large enough.
Also add an assert (only enabled when -DRTE_ENABLE_ASSERT is passed) to
check this constraint.
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Aaron Conole <aconole@redhat.com>