dpdk: false link down issue with ixgbe NIC

This issue is observed with X520-2 NICs on FD.io lab Taishan server.
After VPP booting up and bringing up  the interfaces with command "set
interface state <interface> up", it still shows link down status from
the command "show hardware-interfaces". However, the hardware link
status is actually up. dpdk_process() cannot get the hardware link
status correctly via rte_eth_link_get_nowait().

In ixgbe_dev_link_update_share(), if the media type is fiber and the
link is down, a flag (IXGBE_FLAG_NEED_LINK_CONFIG) is set. A callback to
ixgbe_dev_setup_link_alarm_handler() is scheduled trying to set up the
link and clear the flag afterwards.

If the device is started or stopped before the flag is cleared, the
scheduled callback is canceled. This causes the flag to remain set and
subsequent calls to ixgbe_dev_link_update_share() return without trying
to retrieve the link state because the flag is set.

When the callback is canceled by either interface start or stop
operation, in ixgbe_dev_cancel_link_thread(), after cancelling the
callback/thread, unset the flag on the device to avoid this condition.

Type: fix

Signed-off-by: Lijian Zhang <Lijian.Zhang@arm.com>
Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
Reviewed-by: Jieqiang Wang <Jieqiang.Wang@arm.com>
Change-Id: I04de377dc048307a78a5b7109ebdfaf376d5e029