NFS: Remove the NFSv4 "open optimisation" from nfs_permission

Ever since commit 6168f62c (Add ACCESS operation to OPEN compound)
the NFSv4 atomic open has primed the access cache, and so nfs_permission
will no longer do an RPC call on the wire.

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