Reduce zdb output when pool contains checkpoint

When running zdb without additional arguments against a pool containing
a checkpoint the entire checkpoint spacemap should not be dumped. Make
this behavior conditional upon passing the -mmmm option as described in
the zdb(8) man page.

     -mmmm   Display every spacemap record.

Reviewed-by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7702