mm/shmem.c: clean up comment of shmem_swapin_folio

mm/shmem.c: clean up comment of shmem_swapin_folio

shmem_swapin_folio has changed to use folio but comment still mentions
page.  Update the relevant comment accordingly as suggested by Naoya.

Link: https://lkml.kernel.org/r/20220530115841.4348-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Suggested-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>