NFS: Fix a connectathon regression in NFSv3 and NFSv4

We're failing basic test6 against Linux servers because they lack a correct
change attribute. The fix is to assume that we always want to invalidate
the readdir caches when we call update_changeattr and/or
nfs_post_op_update_inode on a directory.

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