mm/shmem: convert shmem_alloc_and_acct_page to use a folio

Convert shmem_alloc_hugepage() to return the folio that it uses and use a
folio throughout shmem_alloc_and_acct_page().  Continue to return a page
from shmem_alloc_and_acct_page() for now.

Link: https://lkml.kernel.org/r/20220504182857.4013401-22-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>