Check the target function has non-void retval type and set the correct
fetch type if user doesn't specify it.
If the function returns void, $retval is rejected as below;

 # echo 'f unregister_kprobes%return $retval' >> dynamic_events
sh: write error: No such file or directory
 # cat error_log
[   37.488397] trace_fprobe: error: This function returns 'void' type
  Command: f unregister_kprobes%return $retval
                                       ^
Link: https://lore.kernel.org/all/168507476195.913472.16290308831790216609.stgit@mhiramat.roam.corp.google.com/Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>