Set the connection expiry time when a connection becomes idle rather than
doing this in rxrpc_put_connection().  This makes the put path more
efficient (it is likely to be called occasionally whilst a connection has
outstanding calls because active workqueue items needs to be given a ref).

The time is also preset in the connection allocator in case the connection
never gets used.
Signed-off-by: David Howells <dhowells@redhat.com>