RPC: Clean up RPC task structure

Shrink the RPC task structure. Instead of storing separate pointers
 for task->tk_exit and task->tk_release, put them in a structure.

 Also pass the user data pointer as a parameter instead of passing it via
 task->tk_calldata. This enables us to nest callbacks.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/direct.c

@@ -269,7 +269,6 @@ static void nfs_direct_read_schedule(struct nfs_direct_req *dreq,
 
 		data->task.tk_cookie = (unsigned long) inode;
 		data->task.tk_calldata = data;
-		data->task.tk_release = nfs_readdata_release;
 		data->complete = nfs_direct_read_result;
 
 		lock_kernel();