xprtrdma: Boost client's max slot table size to match Linux server

I've heard rumors of an NFS/RDMA server implementation that has a
default credit limit of 1024. The client's default setting remains
at 128.

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