nvlist leaked in zpool_find_config()

nvlist leaked in zpool_find_config()

In `zpool_find_config()`, the `pools` nvlist is leaked.  Part of it (a
sub-nvlist) is returned in `*configp`, but the callers also leak that.

Additionally, in `zdb.c:main()`, the `searchdirs` is leaked.

The leaks were detected by ASAN (`configure --enable-asan`).

This commit resolves the leaks.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #11396