VFS: Allow the filesystem to return a full file pointer on open intent

 This is needed by NFSv4 for atomicity reasons: our open command is in
 fact a lookup+open, so we need to be able to propagate open context
 information from lookup() into the resulting struct file's
 private_data field.

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