zdb: Fix minor memory leak

Commit 6b6aaf6dc2 introduced a small
memory leak in zdb.  This was detected by the LeakSanitizer and was
causing all ztest runs to fail.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #14796

cmd/zdb/zdb.c

@@ -6940,6 +6940,8 @@ dump_block_stats(spa_t *spa)
 		if (dump_opt['b'] >= 2) {
 			dump_size_histograms(zcb);
 		}
+
+		umem_free(mdstats, sizeof (zfs_blkstat_t));
 	}
 
 	(void) printf("\n");