NFS: Don't reset pg_moreio in __nfs_pageio_add_request

Once we've started sending unstable NFS writes, we do not want to
clear pg_moreio, or we may end up sending the very last request as
a stable write if the commit lists are still empty.

Do, however, reset pg_moreio in the case where we end up having to
recoalesce the write if an attempt to use pNFS failed.

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