nfsv4: Switch to generic xattr handling code

This patch make nfsv4 use the generic xattr handling code
to get the nfsv4 acl. This will help us to add richacl
support to nfsv4 in later patches

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>