svcrdma: Increase the server's default RPC/RDMA credit grant

The range of commits from commit e3274026e2ec ("SUNRPC: move all of
xprt handling into svc_xprt_handle()") to commit 15d39883ee7d
("SUNRPC: change the back-channel queue to lwq") enabled NFSD
performance to scale better as the number of nfsd threads is
increased. These commits were merged in v6.7.

Now that the nfsd thread count can scale to more threads, permit
individual clients to make more use of those threads. Increase the
RPC/RDMA per-connection credit grant from 64 to 128 -- same as the
Linux NFS client.

Simple single client fio-based benchmarking so far shows only
improvement, no regression.

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