latency: fix timestamp marking and latency calculation

latency: fix timestamp marking and latency calculation

Latency calculation logic is not correct for the case where
packets gets dropped before TX. As for the dropped packets,
the timestamp is not cleared, and such packets still gets
counted for latency calculation in next runs, that will result
in inaccurate latency measurement.

So fix this issue as below,

Before setting timestamp in mbuf, check mbuf don't have
any prior valid time stamp flag set and after marking
the timestamp, set mbuf flags to indicate timestamp is
valid.

Before calculating timestamp check mbuf flags are set to
indicate timestamp is valid.

With the above logic it is guaranteed that correct timestamps
have been used.

Fixes: 5cd3cac9ed ("latency: added new library for latency stats")
Cc: stable@dpdk.org
Reported-by: Bao-Long Tran <longtb5@viettel.com.vn>
Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
Tested-by: Bao-Long Tran <longtb5@viettel.com.vn>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>