rxrpc: Send jumbo DATA packets

Send jumbo DATA packets if the path-MTU probing using padded PING ACK
packets shows up sufficient capacity to do so.  This allows larger chunks
of data to be sent without reducing the retryability as the subpackets in a
jumbo packet can also be retransmitted individually.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>