Remove pointless memset() in nfsacl_encode().

Thanks to Trond Myklebust <Trond.Myklebust@netapp.com> for pointing out
that it is not needed since posix_acl_init() will set everything
regardless..
Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>