perf evsel: Store backpointer to attached bpf_object

We may want to get to this bpf_object, to search for other BPF programs,
etc.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-3y8hrb6lszjfi23vjlic3cib@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>