zstreamdump: exec zstream dump

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11859

cmd/zstreamdump/zstreamdump

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-zstream dump "$@"
+exec zstream dump "$@"