rxrpc: Update protocol definitions slightly

Update the protocol definitions in include/rxrpc/packet.h slightly:

 (1) Get rid of RXRPC_PROCESS_MAXCALLS as it's redundant (same as
     RXRPC_MAXCALLS).

 (2) In struct rxrpc_jumbo_header, put _rsvd in a union with a field called
     cksum to match struct rxrpc_wire_header.

 (3) Provide RXRPC_JUMBO_SUBPKTLEN which is the total of the amount of data
     in a non-terminal subpacket plus the following secondary header for
     the next packet included in the jumbo packet.

Signed-off-by: David Howells <dhowells@redhat.com>