NFS: Fix the locking in nfs4_callback_getattr

The delegation is protected by RCU now, so we need to replace the
nfsi->rwsem protection with an rcu protected section.

Reported-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>