app/testpmd: fix burst stats reporting

app/testpmd: fix burst stats reporting

When RTE_TEST_PMD_RECORD_BURST_STATS is enabled, testpmd collects
burst statistics and includes them in the port stats report.  The
summary should include top 2 most frequent burst sizes, but there is a
bug in finding the top-2.  During the scan of burst size counts, the
top-2 can change only if top-1 also changes.

Added logic to update the top-2 if current burst size is larger than
existing top-2, but smaller than existing top-1.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Daniel Shelepov <dashel@microsoft.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>