Don't leak accept_fd on thread completion

MFC after:	3 days
Reported by:	Coverity
CID:		1296068
Sponsored by:	Dell EMC Isilon

tests/sys/file/newfileops_on_fork_test.c

@@ -63,6 +63,7 @@ do_accept(__unused void *arg)
 	if (accept_fd < 0)
 		err(1, "accept");
 
+	close(accept_fd);
 	return (NULL);
 }