mm/shmem: add shmem_alloc_folio()

Call vma_alloc_folio() directly instead of alloc_page_vma().  Add a
shmem_alloc_page() wrapper to avoid changing the callers.

Link: https://lkml.kernel.org/r/20220504182857.4013401-21-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>