perf jitdump: Directly mark the jitdump DSO

The DSO being generated was being accessed through a thread's maps,
this is unnecessary as the dso can just be directly found. This avoids
problems with passing a NULL evsel which may be inspected to determine
properties of a callchain when using the buildid DSO marking code.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250724163302.596743-8-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>