Fix: sunrpc: use signed integer for client id

Fix: sunrpc: use signed integer for client id

Within include/linux/sunrpc/clnt.h:struct rpc_cltn, the cl_clid field
is an unsigned integer, which is the type expected by the tracepoint
signature.

However, looking into net/sunrpc/clnt.c:rpc_alloc_clid(), its allocation
considers negative signed integer as errors.

Therefore, in order to properly show "-1" in the trace output (rather
than MAX_INT) when called with a NULL task->tk_client, move to a
signed integer as backing type for the client_id field.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>