shmem: fix obsolete comment in shmem_getpage_gfp()

Replace "fault_mm" with "vmf" in code comment because commit cfda0526
("userfaultfd: shmem: add userfaultfd hook for shared memory faults") has
changed the prototpye of shmem_getpage_gfp() - pass vmf instead of
fault_mm to the function.

Before:
static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
		struct page **pagep, enum sgp_type sgp,
		gfp_t gfp, struct mm_struct *fault_mm, int *fault_type);
After:
static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
		struct page **pagep, enum sgp_type sgp,
		gfp_t gfp, struct vm_area_struct *vma,
		struct vm_fault *vmf, vm_fault_t *fault_type);

Link: http://lkml.kernel.org/r/20190816100204.9781-1-miles.chen@mediatek.comSigned-off-by: Miles Chen <miles.chen@mediatek.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/shmem.c

@@ -1734,7 +1734,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
  * vm. If we swap it in we mark it dirty since we also free the swap
  * entry since a page cannot live in both the swap and page cache.
  *
- * fault_mm and fault_type are only supplied by shmem_fault:
+ * vmf and fault_type are only supplied by shmem_fault:
  * otherwise they are NULL.
  */
 static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,