sunrpc: allow SOMAXCONN backlogged TCP connections

sunrpc: allow SOMAXCONN backlogged TCP connections

The connection backlog passed to listen() denotes the number of
connections that are fully established, but that have not yet been
accept()ed. If the amount goes above that level, new connection requests
will be dropped on the floor until the value goes down. If all the knfsd
threads are bogged down in (e.g.) disk I/O, new connection attempts can
stall because of this.

For the same rationale that Trond points out in the userland patch [1],
ensure that svc_xprt sockets created by the kernel allow SOMAXCONN
(4096) backlogged connections instead of the 64 that they do today.

[1]: https://lore.kernel.org/linux-nfs/20240308180223.2965601-1-trond.myklebust@hammerspace.com/

Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>