mm/shmem: convert shmem_add_to_page_cache to take a folio

Shrinks shmem_add_to_page_cache() by 16 bytes.  All the callers grow,
but this is temporary as they will all be converted to folios soon.

Link: https://lkml.kernel.org/r/20220504182857.4013401-19-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>