NFS: Add missing encode / decode sequence_maxsz to v4.2 operations

These really should have been there from the beginning, but we never
noticed because there was enough slack in the RPC request for the extra
bytes. Chuck's recent patch to use au_cslack and au_rslack to compute
buffer size shrunk the buffer enough that this was now a problem for
SEEK operations on my test client.

Fixes: f4ac1674 ("nfs: Add ALLOCATE support")
Fixes: 2e72448b ("NFS: Add COPY nfs operation")
Fixes: cb95deea ("NFS OFFLOAD_CANCEL xdr")
Fixes: 624bd5b7 ("nfs: Add DEALLOCATE support")
Fixes: 1c6dcbe5

 ("NFS: Implement SEEK")
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>