LoongArch: ftrace: Implement ftrace_find_callable_addr() to simplify code

In the module processing functions, the same logic can be reused by
implementing ftrace_find_callable_addr().

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>