Revert "[PATCH] Fix up mmap_kmem"

This reverts commit 99a10a60ba9bedcf5d70ef81414d3e03816afa3f.

As per Hugh Dickins:

"Nadia Derbey has reported that mmap of /dev/kmem no longer works with
the kernel virtual address as offset, and Franck has confirmed that
his patch came from a misunderstanding of what an offset means to
/dev/kmem - whereas his patch description seems to say that he was
correcting the offset on a few plaforms, there was no such problem to
correct, and his patch was in fact changing its API on all platforms."

Suggested-by: Hugh Dickins <[e-mail]>
Cc: Franck Bui-Huu <[e-mail]>
Cc: Nadia Derbey <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Arjan van de Ven <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/mem.c

···
 {
 	unsigned long pfn;
 
-	/* Turn a pfn offset into an absolute pfn */
-	pfn = PFN_DOWN(virt_to_phys((void *)PAGE_OFFSET)) + vma->vm_pgoff;
+	/* Turn a kernel-virtual address into a physical page frame */
+	pfn = __pa((u64)vma->vm_pgoff << PAGE_SHIFT) >> PAGE_SHIFT;
 
 	/*
 	 * RED-PEN: on some architectures there is more mapped memory