[bcc] fix for new bpf_attach_kprobe signature

bcc changed the signature of bpf_attach_kprobe to accept a new argument:
`maxactive`, which determines the maximum limit of how many calls in
parallel a kretprobe attached to a kernel function can catch. The new
signature is not backwards compatible, thus our build broke. Since
there's no way to feature detect the existance of an argument in a
function using cmake, we use a function pointer with the long signature
and cast bpf_attach_kprobe to it. If we're on an older bcc version, the
last argument will be allocated in a register (according to the
platform's calling convention), but should be ignored inside the
function.

Ref: torvalds/linux@696ced4
Ref: iovisor/bcc#2224
Fixes: #453

src/attached_probe.cpp

@@ -373,10 +373,20 @@ void AttachedProbe::load_prog()
   }
 }
 
+// XXX(mmarchini): bcc changed the signature of bpf_attach_kprobe, adding a new
+// int parameter at the end. Since there's no reliable way to feature-detect
+// this, we create a function pointer with the long signature and cast
+// bpf_attach_kprobe to this function pointer. If we're on an older bcc
+// version, bpf_attach_kprobe call will be augmented with an extra register
+// being used for the last parameter, even though this register won't be used
+// inside the function. Since the register won't be used this is kinda safe,
+// although not ideal.
+typedef int (*attach_probe_wrapper_signature)(int, enum bpf_probe_attach_type, const char*, const char*, uint64_t, int);
+
 void AttachedProbe::attach_kprobe()
 {
-  int perf_event_fd = bpf_attach_kprobe(progfd_, attachtype(probe_.type),
-      eventname().c_str(), probe_.attach_point.c_str(), 0);
+  int perf_event_fd = reinterpret_cast<attach_probe_wrapper_signature>(&bpf_attach_kprobe)(progfd_, attachtype(probe_.type),
+      eventname().c_str(), probe_.attach_point.c_str(), 0, 0);
 
   if (perf_event_fd < 0) {
     if (probe_.orig_name != probe_.name) {