tracing: Cleanup upper_empty() in pid_list

Instead of find_first_bit() use the dedicated bitmap_empty(),
and make upper_empty() a nice one-liner.

While there, fix opencoded BITS_PER_TYPE().

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20250429195119.620204-1-yury.norov@gmail.com


Signed-off-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>