userfaultfd: convert mcontinue_atomic_pte() to use a folio

shmem_getpage() is being replaced by shmem_get_folio() so use a folio
throughout this function.  Saves several calls to compound_head().

Link: https://lkml.kernel.org/r/20220902194653.1739778-33-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>