mm: clarify a confusing comment for remap_pfn_range()

It really made me scratch my head.  Replace the comment with an accurate
and consistent description.

The parameter pfn actually refers to the page frame number which is
right-shifted by PAGE_SHIFT from the physical address.
Signed-off-by: NWANG Wenhu <wenhu.wang@vivo.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Reviewed-by: NAndrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200310073955.43415-1-wenhu.wang@vivo.comSigned-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -1939,7 +1939,7 @@ static inline int remap_p4d_range(struct mm_struct *mm, pgd_t *pgd,
  * remap_pfn_range - remap kernel memory to userspace
  * @vma: user vma to map to
  * @addr: target user address to start at
- * @pfn: physical address of kernel memory
+ * @pfn: page frame number of kernel physical memory address
  * @size: size of map area
  * @prot: page protection flags for this mapping
  *