rxrpc: Separate the packet length from the data length in rxrpc_txbuf

rxrpc: Separate the packet length from the data length in rxrpc_txbuf

Separate the packet length from the data length (txb->len) stored in the
rxrpc_txbuf to make security calculations easier.  Also store the
allocation size as that's an upper bound on the size of the security
wrapper and change a number of fields to unsigned short as the amount of
data can't exceed the capacity of a UDP packet.

Also, whilst we're at it, use kzalloc() for txbufs.

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/20241204074710.990092-11-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>