vm: convert HPET mmap to vm_iomap_memory() helper

This is my example conversion of a few existing mmap users.  The HPET
case is simple, widely available, and easy to test (Clemens Ladisch sent
a trivial test-program for it).
Test-program-by: NClemens Ladisch <clemens@ladisch.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/hpet.c

@@ -373,26 +373,14 @@ static int hpet_mmap(struct file *file, struct vm_area_struct *vma)
 	struct hpet_dev *devp;
 	unsigned long addr;
 
-	if (((vma->vm_end - vma->vm_start) != PAGE_SIZE) || vma->vm_pgoff)
-		return -EINVAL;
-
 	devp = file->private_data;
 	addr = devp->hd_hpets->hp_hpet_phys;
 
 	if (addr & (PAGE_SIZE - 1))
 		return -ENOSYS;
 
-	vma->vm_flags |= VM_IO;
 	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
-
-	if (io_remap_pfn_range(vma, vma->vm_start, addr >> PAGE_SHIFT,
-					PAGE_SIZE, vma->vm_page_prot)) {
-		printk(KERN_ERR "%s: io_remap_pfn_range failed\n",
-			__func__);
-		return -EAGAIN;
-	}
-
-	return 0;
+	return vm_iomap_memory(vma, addr, PAGE_SIZE);
 #else
 	return -ENOSYS;
 #endif