NFSv4: Add 'server capability' flags for NFSv4 recommended attributes

If the NFSv4 server doesn't support a POSIX attribute, the generic NFS code
needs to know that, so that it don't keep trying to poll for it.

However, by the same count, if the NFSv4 server does support that
attribute, then we should ensure that the inode metadata is appropriately
labelled as being untrusted. For instance, if we don't know the correct
value of the file's uid, we should certainly not be caching ACLs or ACCESS
results.

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