NFSv4: Some NFSv4 servers have broken behaviour for the change attribute

The Linux NFSv4 server violates RFC3530 in that the change attribute is not
guaranteed to be updated for every change to the inode. Our optimisation
for checking whether or not the inode metadata has changed or not is broken
too. Grr....

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