tracing: Add warning if string in __assign_str() does not match __string()

In preparation to remove the second parameter of __assign_str(), make sure
it is really a duplicate of __string() by adding a WARN_ON_ONCE().

Link: https://lore.kernel.org/linux-trace-kernel/20240223161356.63b72403@gandalf.local.home

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>