rxrpc: Don't call the tx_ack tracepoint if don't generate an ACK

rxrpc_send_call_packet() is invoking the tx_ack tracepoint before it checks
whether there's an ACK to transmit (another thread may jump in and transmit
it).

Fix this by only invoking the tracepoint if we get a valid ACK to transmit.

Further, only allocate a serial number if we're going to actually transmit
something.

Signed-off-by: David Howells <dhowells@redhat.com>