nfs: handle servers that support only ALLOW ACE type.

Currently we support ACLs if the NFS server file system supports both
ALLOW and DENY ACE types. This patch makes the Linux client work with
ACLs even if the server supports only 'ALLOW' ACE type.

Signed-off-by: Malahal Naineni <malahal@us.ibm.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>