svcrdma: Double the default credit limit

The RDMA credit limit controls how many concurrent RPCs are allowed
per connection.

An NFS/RDMA client and server exchange their credit limits in the
RPC/RDMA headers. The Linux client and the Solaris client and server
allow 32 credits. The Linux server allows only 16, which limits its
performance.

Set the server's default credit limit to 32, like the other well-
known implementations, so the out-of-the-shrinkwrap performance of
the Linux server is better.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>