This change prevents the attempt to add a structure which is
already on the callback list. If a struct with matching
parameters is found on the list, then no action is taken.
Fixes: ac2f69c ("ethdev: fix crash if malloc of user callback fails")
Signed-off-by: E. Scott Daniels <daniels@research.att.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>