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>
nfs_page_find_request_locked(struct nfs_inode *nfsi, struct page *page)
{
	struct nfs_page *req = NULL;

	if (PagePrivate(page))
		req = (struct nfs_page *)page_private(page);
	else if (unlikely(PageSwapCache(page))) {
		struct nfs_page *freq, *t;

		/* Linearly search the commit list for the correct req */
		list_for_each_entry_safe(freq, t, &nfsi->commit_info.list, wb_list) {
			if (freq->wb_page == page) {
				req = freq;
				break;
			}
		}
	}

	if (req)
		kref_get(&req->wb_kref);

	return req;
}

nfs_page_find_request_locked(struct nfs_inode *nfsi, struct page *page)
{
	struct nfs_page *req = NULL;

	if (PagePrivate(page))
		req = (struct nfs_page *)page_private(page);
	else if (unlikely(PageSwapCache(page))) {
		struct nfs_page *freq, *t;

		/* Linearly search the commit list for the correct req */
		list_for_each_entry_safe(freq, t, &nfsi->commit_info.list, wb_list) {
			if (freq->wb_page == page) {
				req = freq;
				break;
			}
		}
	}

	if (req)
		kref_get(&req->wb_kref);

	return req;
}
