nfs4_setup_readdir(): insufficient locking for ->d_parent->d_inode dereferencing

nfs4_setup_readdir(): insufficient locking for ->d_parent->d_inode dereferencing

Theoretically it's an oopsable race, but I don't believe one can manage
to hit it on real hardware; might become doable on a KVM, but it still
won't be easy to attack.

Anyway, it's easy to deal with - since xdr_encode_hyper() is just a call of
put_unaligned_be64(), we can put that under ->d_lock and be done with that.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>