ACPI: Fix wrong parameter passed to memblock_reserve

Commit 53aac44c (ACPI: Store valid ACPI tables passed via early initrd
in reserved memblock areas) introduced acpi_initrd_override() that
passes a wrong value as the second argument to memblock_reserve().

Namely, the second argument of memblock_reserve() is the size of the
region, not the address of the top of it, so make
acpi_initrd_override() pass the size in there as appropriate.

[rjw: Changelog]
Signed-off-by: Wang YanQing <udknight@gmail.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Cc: 3.8+ <stable@vger.kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>