NFS: Fix nfs_post_op_update_inode() to set an attribute barrier

nfs_post_op_update_inode() is called after a self-induced attribute
update. Ensure that it also sets the barrier.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Tested-by: Chuck Lever <chuck.lever@oracle.com>