SUNRPC: Add correct rcu_dereference annotation in rpc_clnt_set_transport

rpc_clnt_set_transport should use rcu_derefence_protected(), as it is
only safe to be called with the rpc_clnt::cl_lock held.

Cc: Chuck Lever <Chuck.Lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>