NFSv4: Dont list system.nfs4_acl for filesystems that don't support it.

Thanks to Frank Filz for pointing out that we list system.nfs4_acl extended
attribute even on filesystems where we don't actually support nfs4_acl.
This is inconsistent with the e.g. ext3 POSIX ACL behaviour, and seems to
annoy cp.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>