KVM: MMU: Fix host memory corruption on i386 with >= 4GB ram

PAGE_MASK is an unsigned long, so using it to mask physical addresses on
i386 (which are 64-bit wide) leads to truncation.  This can result in
page->private of unrelated memory pages being modified, with disasterous
results.

Fix by not using PAGE_MASK for physical addresses; instead calculate
the correct value directly from PAGE_SIZE.  Also fix a similar BUG_ON().

Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Avi Kivity <avi@qumranet.com>