Revert "fs: nfs: fix missing refcnt by replacing folio_set_private by folio_attach_private"

This reverts commit 03e02b94.

As was pointed out during code review, there is no need to use
folio_attach_private()/folio_detach_private() when a refcount to the
folio is already carried by the struct nfs_page.

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