NFS: fix up nfs_release_folio() to try to release the page

If the gfp context allows it, and we're not kswapd, then try to write
out the folio that has private data.

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