arm64: mm: add __init section marker to free_initrd_mem

It is not needed after booting, this patch moves the
free_initrd_mem() function to the __init section.

This patch also make keep_initrd __initdata, to reduce kernel
size.
Signed-off-by: NWang Long <long.wanglong@huawei.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/mm/init.c

@@ -358,9 +358,9 @@ void free_initmem(void)
 
 #ifdef CONFIG_BLK_DEV_INITRD
 
-static int keep_initrd;
+static int keep_initrd __initdata;
 
-void free_initrd_mem(unsigned long start, unsigned long end)
+void __init free_initrd_mem(unsigned long start, unsigned long end)
 {
 	if (!keep_initrd)
 		free_reserved_area((void *)start, (void *)end, 0, "initrd");