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>

Author: Al Viro

fs/nfs/nfs4proc.c

@@ -391,7 +391,9 @@ static void nfs4_setup_readdir(u64 cookie, __be32 *verifier, struct dentry *dent
 	*p++ = htonl(attrs);                           /* bitmap */
 	*p++ = htonl(12);             /* attribute buffer length */
 	*p++ = htonl(NF4DIR);
+	spin_lock(&dentry->d_lock);
 	p = xdr_encode_hyper(p, NFS_FILEID(d_inode(dentry->d_parent)));
+	spin_unlock(&dentry->d_lock);
 
 	readdir->pgbase = (char *)p - (char *)start;
 	readdir->count -= readdir->pgbase;