Add nvlist payload stats for zdb -ll dump

When dumping the ZFS vdev label with 'zdb -ll', also dump
some nvlist stats to help analyze the current footprint.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Don Brady <don.brady@intel.com>
Closes #5724