zvol_wait logic may terminate prematurely

Setups that have a lot of zvols may see zvol_wait terminate prematurely
even though the script is still making progress. For example, we have a
customer that called zvol_wait for ~7100 zvols and by the last iteration
of that script it was still waiting on ~2900. Similarly another one
called zvol_wait for 2200 and by the time the script terminated there
were only 50 left.

This patch adjusts the logic to stay within the outer loop of the script
if we are making any progress whatsoever.

Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #13998