selftests/bpf: Fix uprobe consumer test

selftests/bpf: Fix uprobe consumer test

With newly merged code the uprobe behaviour is slightly different
and affects uprobe consumer test.

We no longer need to check if the uprobe object is still preserved
after removing last uretprobe, because it stays as long as there's
pending/installed uretprobe instance.

This allows to run uretprobe consumers registered 'after' uprobe was
hit even if previous uretprobe got unregistered before being hit.

The uprobe object will be now removed after the last uprobe ref is
released and in such case it's held by ri->uprobe (return instance)
which is released after the uretprobe is hit.

Reported-by: Ihor Solodrai <ihor.solodrai@pm.me>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Ihor Solodrai <ihor.solodrai@pm.me>
Closes: https://lore.kernel.org/bpf/w6U8Z9fdhjnkSp2UaFaV1fGqJXvfLEtDKEUyGDkwmoruDJ_AgF_c0FFhrkeKW18OqiP-05s9yDKiT6X-Ns-avN_ABf0dcUkXqbSJN1TQSXo=@pm.me/
Signed-off-by: Alexei Starovoitov <ast@kernel.org>