x86: check mptable physptr with max_low_pfn on 32bit

Impact: fix early crash on LinuxBIOS systems

Kevin O'Connor reported that Coreboot aka LinuxBIOS tries to put
mptable somewhere very high, well above max_low_pfn (below which
BIOSes generally put the mptable), causing a panic.

The BIOS will probably be changed to be compatible with older
Linus versions, but nevertheless the MP-spec does not forbid
an MP-table in arbitrary system RAM, so make sure it all
works even if the table is in an unexpected place.

Check physptr with max_low_pfn * PAGE_SIZE.

Reported-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Stefan Reinauer <stepan@coresystems.de>
Cc: coreboot@coreboot.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>