ftrace: Avoid needless updates of the ftrace function call

ftrace: Avoid needless updates of the ftrace function call

Song Shuai reported:

    The list func (ftrace_ops_list_func) will be patched first
    before the transition between old and new calls are set,
    which fixed the race described in this commit `59338f75`.

    While ftrace_trace_function changes from the list func to a
    ftrace_ops func, like unregistering the klp_ops to leave the only
    global_ops in ftrace_ops_list, the ftrace_[regs]_call will be
    replaced with the list func although it already exists. So there
    should be a condition to avoid this.

And suggested using another variable to keep track of what the ftrace
function is set to. But this could be simplified by using a helper
function that does the same with a static variable.

Link: https://lore.kernel.org/lkml/20221026132039.2236233-1-suagrfillet@gmail.com/
Link: https://lore.kernel.org/linux-trace-kernel/20221122180905.737b6f52@gandalf.local.home
Reported-by: Song Shuai <suagrfillet@gmail.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>