bpf: Force uprobe bpf program to always return 0

As suggested by Andrii make uprobe multi bpf programs to always return 0,
so they can't force uprobe removal.

Keeping the int return type for uprobe_prog_run, because it will be used
in following session changes.

Fixes: 89ae89f5 ("bpf: Add multi uprobe link")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241108134544.480660-3-jolsa@kernel.org