net/tap: fix blocked Rx packets

net/tap: fix blocked Rx packets

When OS sends more packets than are being read with a single
'rte_eth_rx_burst' call, rx packets are getting stucked in the tap pmd
and are unable to receive, because trigger_seen is getting updated
and consecutive calls are not getting any packets.

Do not update trigger_seen unless less than a max number of packets were
received allowing next call to receive the rest.

Remove unnecessary compiler barrier.

Fixes: a0d8e807d9de ("net/tap: add Rx trigger")
Cc: stable@dpdk.org
Signed-off-by: Marcin Smoczynski <marcinx.smoczynski@intel.com>
Tested-by: Mariusz Drost <mariuszx.drost@intel.com>
Tested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Reviewed-by: Keith Wiles <keith.wiles@intel.com>