rxrpc: Ask the security class how much space to allow in a packet

rxrpc: Ask the security class how much space to allow in a packet

Ask the security class how much header and trailer space to allow for when
allocating a packet, given how much data is remaining.

This will allow the rxgk security class to stick both a trailer in as well
as a header as appropriate in the future.

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