Taught zdb -bb to print metadata totals

People often want estimates of how much of their pool is occupied
by metadata, but they end up using lots of text processing on zdb's
output to get it.

So let's just...provide it for them.

Now, zdb -bbbs will output something like:

Blocks LSIZE PSIZE ASIZE avg comp %Total Type
[...]

68  1.06M    272K    544K      8K    4.00     0.00      L6 Total

1.71K 212M 6.85M 13.7M 8K 30.91 0.00 L5 Total
1.71K 212M 6.85M 13.7M 8K 30.91 0.00 L4 Total
1.73K 214M 6.92M 13.8M 8K 30.89 0.00 L3 Total
18.7K 2.29G 111M 221M 11.8K 21.19 0.00 L2 Total
3.56M 454G 28.4G 56.9G 16.0K 15.97 0.19 L1 Total

308M  36.8T   28.2T   28.6T   95.1K    1.30    99.80      L0 Total
311M  37.3T   28.3T   28.6T   94.2K    1.32   100.00  Total

50.4M 774G 113G 291G 5.77K 6.85 0.99 Metadata Total

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #14746