examples/multi_process: fix resilience by enabling Rx drop

examples/multi_process: fix resilience by enabling Rx drop

The symmetric_mp example app is set up to allow two processes to
share a NIC port, with each pulling packets from one queue. In order
to have the app continue working when one of the process dies, the
drop_en bit should be set in the NIC configuration. Without this bit
set, the NIC will stall once any queue fills. With the bit set, once
a queue fills, all subsequent packets for that queue are discarded
allowing other queues to continue operating as normal.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>