Fix zpool iostat bandwidth/ops calculation

print_vdev_stats() subtracts the old bandwidth/ops stats from the new stats
to calculate the bandwidth/ops numbers in "zpool iostat". However when the
TXG numbers change between stats, zpool_refresh_stats() will incorrectly assign
a NULL to the old stats. This causes print_vdev_stats() to use zeroes for
the old bandwidth/ops numbers, resulting in an inaccurate calculation.

This fix allows the calculation to happen even when TXGs change.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4387