ftrace: Remove data field from ftrace_func_probe structure

No users of the function probes uses the data field anymore. Remove it, and
change the init function to take a void *data parameter instead of a
void **data, because the init will just get the data that the registering
function was received, and there's no state after it is called.

The other functions for ftrace_probe_ops still take the data parameter, but
it will currently only be passed NULL. It will stay as a parameter for
future data to be passed to these functions.

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