ftrace: Use LIST_HEAD to initialize clear_hash

Use LIST_HEAD() to initialize clear_hash instead of open-coding it.

Link: https://lore.kernel.org/linux-trace-kernel/20230809071551.913041-1-ruanjinjie@huawei.com

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>