nios2: switch to kernel_clone()

The old _do_fork() helper is removed in favor of the new kernel_clone() helper.
The latter adheres to naming conventions for kernel internal syscall helpers.
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Ley Foon Tan <ley.foon.tan@intel.com>
Link: https://lore.kernel.org/r/20200819104655.436656-6-christian.brauner@ubuntu.com

arch/nios2/kernel/process.c

@@ -266,5 +266,5 @@ asmlinkage int nios2_clone(unsigned long clone_flags, unsigned long newsp,
 		.tls		= tls,
 	};
 
-	return _do_fork(&args);
+	return kernel_clone(&args);
 }