x86/setup: Make relocated_ramdisk a local variable of relocate_initrd()

After

  0b62f6cb ("x86/microcode/32: Move early loading after paging enable"),

the global variable relocated_ramdisk is no longer used anywhere except
for the relocate_initrd() function. Make it a local variable of that
function.

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/r/20231113034026.130679-1-ytcoode@gmail.com