l2tp: add tracepoint definitions in trace.h

l2tp can provide a better debug experience using tracepoints rather than
printk-style logging.

Add tracepoint definitions in trace.h for use in the l2tp subsystem
code.

Add preprocessor definitions for the length of session and tunnel names
in l2tp_core.h so we can reuse these in trace.h.

Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>