fix bogon in /dev/mem mmap'ing on nommu

While digging through my MAP_FIXED changes, I found that rather obvious
bug in /dev/mem mmap implementation for nommu archs. get_unmapped_area()
is expected to return an address, not a pfn.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-By: David Howells <dhowells@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/mem.c

@@ -248,7 +248,7 @@ static unsigned long get_unmapped_area_mem(struct file *file,
 {
 	if (!valid_mmap_phys_addr_range(pgoff, len))
 		return (unsigned long) -EINVAL;
-	return pgoff;
+	return pgoff << PAGE_SHIFT;
 }
 
 /* can't do an in-place private mapping if there's no MMU */