bpftask: return exit code

include/linux/bpf_types.h

include/linux/syscalls.h

include/uapi/linux/bpf.h

kernel/bpf/task.c

scripts/bpf_helpers_doc.py

tools/include/uapi/linux/bpf.h

tools/lib/bpf/libbpf.c

tools/lib/bpf/libbpf_probes.c