NFS: Remove bogus check of cache_change_attribute in nfs_update_inode

Remove the bogus 'data_stable' check in nfs_update_inode. The
cache_change_attribute tells you if the directory changed on the server,
and should have nothing to do with the file length.

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