rxrpc: Fix ACK generation from the connection event processor

Repeat terminal ACKs and now terminal ACKs are now generated from the
connection event processor rather from call handling as this allows us to
discard client call structures as soon as possible and free up the channel
for a follow on call.

However, in ACKs so generated, the additional information trailer is
malformed because the padding that's meant to be in the middle isn't
included in what's transmitted.

Fix it so that the 3 bytes of padding are included in the transmission.

Further, the trailer is misaligned because of the padding, so assigment to
the u16 and u32 fields inside it might cause problems on some arches, so
fix this by breaking the padding and the trailer out of the packed struct.

(This also deals with potential compiler weirdies where some of the nested
structs are packed and some aren't).

The symptoms can be seen in wireshark as terminal DUPLICATE or IDLE ACK
packets in which the Max MTU, Interface MTU and rwind fields have weird
values and the Max Packets field is apparently missing.

Reported-by: Jeffrey Altman <jaltman@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>