ftrace: Mark get_lock_parent_ip() __always_inline

If the compiler decides not to inline this function then preemption
tracing will always show an IP inside the preemption disabling path and
never the function actually calling preempt_{enable,disable}.

Link: https://lore.kernel.org/linux-trace-kernel/20230327173647.1690849-1-john@metanate.com

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: stable@vger.kernel.org
Fixes: f904f582

 ("sched/debug: Fix preempt_disable_ip recording for preempt_disable()")
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>