Check the return value in clonefile test

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Signed-off-by: Kay Pedersen <mail@mkwg.de>
Closes #15128

tests/zfs-tests/cmd/clonefile.c

@@ -212,7 +212,7 @@ main(int argc, char **argv)
 
 	int dfd = open(argv[optind+1], O_WRONLY|O_CREAT,
 	    S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH);
-	if (sfd < 0) {
+	if (dfd < 0) {
 		fprintf(stderr, "open: %s: %s\n",
 		    argv[optind+1], strerror(errno));
 		close(sfd);