nfs: don't return AOP_WRITEPAGE_ACTIVATE from nfs_do_writepage

nfs_do_writepage is a successful return that requires the caller to
unlock the folio.  Using it here requires special casing both in
nfs_do_writepage and nfs_writepages_callback and leaves a land mine in
nfs_wb_folio in case it ever set the flag.  Remove it and just
unconditionally unlock in nfs_writepages_callback.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>