bpf: Allow kfunc in tracing and syscall programs.

Tracing and syscall BPF program types are very convenient to add BPF
capabilities to subsystem otherwise not BPF capable.
When we add kfuncs capabilities to those program types, we can add
BPF features to subsystems without having to touch BPF core.

Signed-off-by: Benjamin Tissoires <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

kernel/bpf/btf.c

···
 	BTF_KFUNC_HOOK_XDP,
 	BTF_KFUNC_HOOK_TC,
 	BTF_KFUNC_HOOK_STRUCT_OPS,
+	BTF_KFUNC_HOOK_TRACING,
+	BTF_KFUNC_HOOK_SYSCALL,
 	BTF_KFUNC_HOOK_MAX,
 };
 
···
 		return BTF_KFUNC_HOOK_TC;
 	case BPF_PROG_TYPE_STRUCT_OPS:
 		return BTF_KFUNC_HOOK_STRUCT_OPS;
+	case BPF_PROG_TYPE_TRACING:
+		return BTF_KFUNC_HOOK_TRACING;
+	case BPF_PROG_TYPE_SYSCALL:
+		return BTF_KFUNC_HOOK_SYSCALL;
 	default:
 		return BTF_KFUNC_HOOK_MAX;
 	}