x86: reserve exact size of mptable

Impact: save a bit of RAM

Get the exact size for the reserve_bootmem() call.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <49AE4922.605@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>