stat: make priority summary statistics consistent with percentiles

stat: make priority summary statistics consistent with percentiles

When lat_percentiles=1, reported high/low priority percentiles will
describe total latency values. Make the summary statistics (min, max,
mean, stddev) also describe total latency. Otherwise there will be a
mismatch in the JSON output with completion latency summary statistics
combined with total latency percentile values.

Signed-off-by: Vincent Fu <vincent.fu@wdc.com>