NFS: Convert the function nfs_wb_page() to use folios

Convert to use the folio functions, but pass the struct page to
nfs_writepage_locked() for now.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>