x86/kvm: Handle async page faults directly through do_page_fault()

KVM overloads #PF to indicate two types of not-actually-page-fault
events.  Right now, the KVM guest code intercepts them by modifying
the IDT and hooking the #PF vector.  This makes the already fragile
fault code even harder to understand, and it also pollutes call
traces with async_page_fault and do_async_page_fault for normal page
faults.

Clean it up by moving the logic into do_page_fault() using a static
branch.  This gets rid of the platform trap_init override mechanism
completely.

[ tglx: Fixed up 32bit, removed error code from the async functions and
  	massaged coding style ]

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200505134059.169270470@linutronix.de

arch/x86/entry/entry_32.S

@@ -1693,14 +1693,6 @@ SYM_CODE_START(general_protection)
 	jmp	common_exception
 SYM_CODE_END(general_protection)
 
-#ifdef CONFIG_KVM_GUEST
-SYM_CODE_START(async_page_fault)
-	ASM_CLAC
-	pushl	$do_async_page_fault
-	jmp	common_exception_read_cr2
-SYM_CODE_END(async_page_fault)
-#endif
-
 SYM_CODE_START(rewind_stack_do_exit)
 	/* Prevent any naive code from trying to unwind to our caller. */
 	xorl	%ebp, %ebp