x86/mm: Remove spurious fault pkey check

Spurious faults only ever occur in the kernel's address space.  They
are also constrained specifically to faults with one of these error codes:

	X86_PF_WRITE | X86_PF_PROT
	X86_PF_INSTR | X86_PF_PROT

So, it's never even possible to reach spurious_kernel_fault_check() with
X86_PF_PK set.

In addition, the kernel's address space never has pages with user-mode
protections.  Protection Keys are only enforced on pages with user-mode
protection.

This gives us lots of reasons to not check for protection keys in our
sprurious kernel fault handling.

But, let's also add some warnings to ensure that these assumptions about
protection keys hold true.

Cc: x86@kernel.org
Cc: Jann Horn <jannh@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180928160231.243A0D6A@viggo.jf.intel.com