zdb: add --class=(normal|special|...) to filter blocks by alloc class

When counting blocks to generate block size histograms (-bb), accept a
--class= argument (as a comma-separated list of either "normal",
"special", "dedup" or "other") to only consider blocks that belong to
these metaslab classes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ivan Shapovalov <intelfx@intelfx.name>
Closes #16999