tracing: avoid warnings from zero-arg tracepoints

Tracepoints with no arguments can issue two warnings:

	"field" defined by not used
	"ret" is uninitialized in this function

Mark field as being OK to leave unused, and initialize ret.

[ Impact: fix false positive compiler warnings. ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: mathieu.desnoyers@polymtl.ca
LKML-Reference: <1239950139-1119-5-git-send-email-jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>