event: add function for reading unlink in progress

event: add function for reading unlink in progress

This commit introduces a new function in the eventdev API,
which allows applications to read the number of unlink requests
in progress on a particular port of an eventdev instance.

This information allows applications to verify when no more packets
from a particular queue (or any queue) will arrive at a port.
The application could decide to stop polling, or put the core into
a sleep state if it wishes, as it is ensured that no new packets
will arrive at a particular port anymore if all queues are unlinked.

Suggested-by: Matias Elo <matias.elo@nokia.com>
Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>