rxrpc: uninitialized variable in rxrpc_send_ack_packet()

The "pkt" was supposed to have been deleted in a previous patch.  It
leads to an uninitialized variable bug.

Fixes: 72f0c6fb ("rxrpc: Allocate ACK records at proposal and queue for transmission")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>