examples/ipsec-secgw: cleanup worker state before exit

examples/ipsec-secgw: cleanup worker state before exit

Event ports are configured to implicitly release the scheduler contexts
currently held in the next call to rte_event_dequeue_burst().
A worker core might still hold a scheduling context during exit as the
next call to rte_event_dequeue_burst() is never made.
This might lead to deadlock based on the worker exit timing and when
there are very less number of flows.

Add a cleanup function to release any scheduling contexts held by the
worker by using RTE_EVENT_OP_RELEASE.

Signed-off-by: Pavan Nikhilesh <pbhagavatula@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>