nfs: nfs4xdr: encode_compound_hdr does not have to round up reserved bytes

This is already done by xdr_reserve_space and since encode_compound_hdr
is adding a byte count to "12" which is already word aligned, the xdr
level rounding will work just as well.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>