tracing: Use flexible array for entry fetch code

tracing: Use flexible array for entry fetch code

Store probe entry fetch instructions in the probe_entry_arg
allocation instead of allocating a separate instruction array.

This keeps the entry fetch code tied to the entry argument lifetime while
leaving regular probe_arg instruction arrays separately allocated and
freed.

Assisted-by: Codex:GPT-5.5
Link: https://lore.kernel.org/all/20260520215817.16560-1-rosenp@gmail.com/
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>