rxrpc: Implement path-MTU probing using padded PING ACKs (RFC8899)

rxrpc: Implement path-MTU probing using padded PING ACKs (RFC8899)

Implement path-MTU probing (along the lines of RFC8899) by padding some of
the PING ACKs we send.  PING ACKs get their own individual responses quite
apart from the acking of data (though, as ACKs, they fulfil that role
also).

The probing concentrates on packet sizes that correspond how many
subpackets can be stuffed inside a jumbo packet as jumbo DATA packets are
just aggregations of individual DATA packets and can be split easily for
retransmission purposes.

If we want to perform probing, we advertise this by setting the maximum
number of jumbo subpackets to 0 in the ack trailer when we send an ACK and
see if the peer is also advertising the service.  This is interpreted by
non-supporting Rx stacks as an indication that jumbo packets aren't
supported.

The MTU sizes advertised in the ACK trailer AF_RXRPC transmits are pegged
at a maximum of 1444 unless pmtud is supported by both sides.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20241204074710.990092-10-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>