nfs: remove nfs_folio_private_request

nfs_folio_private_request is a trivial wrapper around, which itself has
fallen out of favor and has been replaced with plain ->private
dereferences in recent folio conversions.  Do the same for nfs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>