xprtrdma: Enable pad optimization

The Linux NFS/RDMA server used to reject NFSv3 WRITE requests when
pad optimization was enabled. That bug was fixed by commit
e560e3b5

 ("svcrdma: Add zero padding if the client doesn't send
it").

We can now enable pad optimization on the client, which helps
performance and is supported now by both Linux and Solaris servers.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>