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

cmd/zhack/zhack.c

@@ -150,6 +150,7 @@ zhack_import(char *target, boolean_t readonly)
 	zfeature_checks_disable = B_TRUE;
 	error = spa_import(target, config, props,
 	    (readonly ?  ZFS_IMPORT_SKIP_MMP : ZFS_IMPORT_NORMAL));
+	fnvlist_free(config);
 	zfeature_checks_disable = B_FALSE;
 	if (error == EEXIST)
 		error = 0;