x86: fix alloc_mptable()

Fix the conditions when we stop updating the mptable due to
running out of slots.

[ Impact: fix memory corruption / non-working update_mptable boot parameter ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Len Brown <lenb@kernel.org>
LKML-Reference: <4A01C3BB.1000609@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>