ftrace_graph_ret_addr() can be called by stack unwinding code to convert
a found stack return address ('ret') to its original value, in case the
function graph tracer has modified it to be 'return_to_handler'. If the
hasn't been modified, the unchanged value of 'ret' is returned.
Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>