tracing: Rename ftrace_data_offset_##call to trace_event_data_offset_##call

The name "ftrace" really refers to the function hook infrastructure. It
is not about the trace_events. The structure ftrace_data_offset_##call is
used to find the offsets of dynamically allocated fields in trace_events.
It has nothing to do with function tracing. Rename it.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>