tracing: probe-events: Cleanup entry-arg storing code

Cleanup __store_entry_arg() so that it is easier to understand.
The main complexity may come from combining the loops for finding
stored-entry-arg and max-offset and appending new entry.

This split those different loops into 3 parts, lookup the same
entry-arg, find the max offset and append new entry.

Link: https://lore.kernel.org/all/174323039929.348535.4705349977127704120.stgit@devnote2/



Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>