x86: make spurious fault handler aware of large mappings

In very rare cases, on certain CPUs, we could end up in the spurious
fault handler and ignore a large pud/pmd mapping. The resulting pte
pointer points into the mapped physical space and dereferencing it
will fault recursively.

Make the code aware of large mappings and do the permission check
on the pmd/pud entry, when a large pud/pmd mapping is detected.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>