Zero struct for zdb dump_block_stats

Accidentally dropped the zeroing of this structure in the
gcc-missing-braces topic branch which was causing a fall positive
space leak in ztest.  Ensure the structure is zero'ed before use.

cmd/zdb/zdb.c

@@ -2179,6 +2179,7 @@ dump_block_stats(spa_t *spa)
 	 * it's not part of any space map) is a double allocation,
 	 * reference to a freed block, or an unclaimed log block.
 	 */
+	bzero(&zcb, sizeof(zdb_cb_t));
 	zdb_leak_init(spa, &zcb);
 
 	/*