x86: decouple pagefault-logic in highmem/kmap

With the separation of pagefault_{disable,enable}() from the
preempt_count a previously overlooked dependancy became painfully
clear.

kmap_atomic() is per cpu and relies not only on disabling the
pagefault handler, but really needs preemption disabled too.

Make this explicit now - so that we can change pagefault_disable().
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/mm/highmem_32.c

@@ -33,6 +33,7 @@ void *kmap_atomic_prot(struct page *page, enum km_type type, pgprot_t prot)
 	unsigned long vaddr;
 
 	/* even !CONFIG_PREEMPT needs this, for in_atomic in do_page_fault */
+	preempt_disable();
 	pagefault_disable();
 
 	if (!PageHighMem(page))
@@ -74,6 +75,7 @@ void kunmap_atomic(void *kvaddr, enum km_type type)
 	}
 
 	pagefault_enable();
+	preempt_enable();
 }
 
 /*