tracing: Rename ftrace_get_offsets_##call() to trace_event_get_offsets_##call()

The name "ftrace" really refers to the function hook infrastructure. It
is not about the trace_events. The function ftrace_get_offsets_##call()
is used to find the offset into dynamically allocated trace event fields
for printing. It has nothing to do with function tracing. Rename it.

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