arm64: Use generic kernel_thread() implementation

This patch enables CONFIG_GENERIC_KERNEL_THREAD on arm64, changes
copy_threads to cope with kernel threads creation and adapts
ret_from_fork accordingly. The arm64-specific kernel_thread
implementation is no longer needed.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index a6f3f7d..08db897 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -611,7 +611,10 @@
  */
 ENTRY(ret_from_fork)
 	bl	schedule_tail
-	get_thread_info tsk
+	cbz	x19, 1f				// not a kernel thread
+	mov	x0, x20
+	blr	x19
+1:	get_thread_info tsk
 	b	ret_to_user
 ENDPROC(ret_from_fork)