nfs: always make sure page is up-to-date before extending a write to cover the entire page

We should always make sure the cached page is up-to-date when we're
determining whether we can extend a write to cover the full page -- even
if we've received a write delegation from the server.

Commit c7559663 added logic to skip this check if we have a write
delegation, which can lead to data corruption such as the following
scenario if client B receives a write delegation from the NFS server:

Client A:
    # echo 123456789 > /mnt/file

Client B:
    # echo abcdefghi >> /mnt/file
    # cat /mnt/file
    0�D0�abcdefghi

Just because we hold a write delegation doesn't mean that we've read in
the entire page contents.

Cc: <stable@vger.kernel.org> # v3.11+
Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
void nfs_writedata_release(struct nfs_write_data *wdata)
{
	struct nfs_pgio_header *hdr = wdata->header;
	struct nfs_write_header *write_header = container_of(hdr, struct nfs_write_header, header);

	put_nfs_open_context(wdata->args.context);
	if (wdata->pages.pagevec != wdata->pages.page_array)
		kfree(wdata->pages.pagevec);
	if (wdata == &write_header->rpc_data) {
		wdata->header = NULL;
		wdata = NULL;
	}
	if (atomic_dec_and_test(&hdr->refcnt))
		hdr->completion_ops->completion(hdr);
	/* Note: we only free the rpc_task after callbacks are done.
	 * See the comment in rpc_free_task() for why
	 */
	kfree(wdata);
}

void nfs_writedata_release(struct nfs_write_data *wdata)
{
	struct nfs_pgio_header *hdr = wdata->header;
	struct nfs_write_header *write_header = container_of(hdr, struct nfs_write_header, header);

	put_nfs_open_context(wdata->args.context);
	if (wdata->pages.pagevec != wdata->pages.page_array)
		kfree(wdata->pages.pagevec);
	if (wdata == &write_header->rpc_data) {
		wdata->header = NULL;
		wdata = NULL;
	}
	if (atomic_dec_and_test(&hdr->refcnt))
		hdr->completion_ops->completion(hdr);
	/* Note: we only free the rpc_task after callbacks are done.
	 * See the comment in rpc_free_task() for why
	 */
	kfree(wdata);
}
