MFC r329694: MFV r324198: 8081 Compiler warnings in zdb

illumos/illumos-gate@3f7978d02b206a6ebc5652c91aa9f42da6fbe00c
https://github.com/illumos/illumos-gate/commit/3f7978d02b206a6ebc5652c91aa9f42da6fbe00c

https://www.illumos.org/issues/8081

zdb(8) is full of minor problems that generate compiler warnings. On FreeBSD,
which uses -WError, the only way to build it is to disable all compiler
warnings. This makes it much harder to detect newly introduced bugs. We should
cleanup all the warnings.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Alan Somers <asomers@gmail.com>