NFS: Ensure we always dereference the page head last

This fixes a race with nfs_page_group_sync_on_bit() whereby the
call to wake_up_bit() in nfs_page_group_unlock() could occur after
the page header had been freed.

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

fs/nfs/pagelist.c

@@ -306,14 +306,11 @@ static void
 nfs_page_group_destroy(struct kref *kref)
 {
 	struct nfs_page *req = container_of(kref, struct nfs_page, wb_kref);
+	struct nfs_page *head = req->wb_head;
 	struct nfs_page *tmp, *next;
 
-	/* subrequests must release the ref on the head request */
-	if (req->wb_head != req)
-		nfs_release_request(req->wb_head);
-
 	if (!nfs_page_group_sync_on_bit(req, PG_TEARDOWN))
-		return;
+		goto out;
 
 	tmp = req;
 	do {
@@ -324,6 +321,10 @@ nfs_page_group_destroy(struct kref *kref)
 		nfs_free_request(tmp);
 		tmp = next;
 	} while (tmp != req);
+out:
+	/* subrequests must release the ref on the head request */
+	if (head != req)
+		nfs_release_request(head);
 }
 
 /**