tcptracer: initialize skc_net variable

[iaguis]

We need to do this in old kernels since the verifier was stricter then.

Tested with 4.4.105.

Fixes #1482

[yonghong-song]

[buildbot, ok to test]

[yonghong-song]

LGTM

[brendangregg]

Errors on 4.9-rc2:

./tcptracer.py.2
bpf: Permission denied
0: (79) r7 = *(u64 *)(r1 +80)
1: (85) call 14
2: (7b) *(u64 *)(r10 -8) = r0
3: (18) r1 = 0x4e0d6540
5: (bf) r2 = r10
6: (07) r2 += -8
7: (85) call 1
8: (bf) r6 = r0
9: (15) if r6 == 0x0 goto pc+55
 R0=map_value_or_null(ks=8,vs=8),min_value=0,max_value=0 R6=map_value(ks=8,vs=8),min_value=0,max_value=0 R7=inv R10=fp
10: (18) r1 = 0x4e0d6540
12: (bf) r2 = r10
13: (07) r2 += -8
14: (85) call 3
15: (67) r7 <<= 32
16: (77) r7 >>= 32
17: (55) if r7 != 0x0 goto pc+47
 R0=inv,min_value=0,max_value=0 R6=map_value(ks=8,vs=8),min_value=0,max_value=0 R7=inv32,min_value=0,max_value=0 R10=fp
18: (79) r1 = *(u64 *)(r6 +0)
19: (b7) r2 = 0
20: (7b) *(u64 *)(r10 -16) = r2
21: (7b) *(u64 *)(r10 -24) = r2
22: (63) *(u32 *)(r10 -48) = r2
23: (69) r6 = *(u16 *)(r1 +12)
R1 invalid mem access 'inv'

HINT: The invalid mem access 'inv' error can happen if you try to dereference memory without first using bpf_probe_read() to copy it to the BPF stack. Sometimes the bpf_probe_read is automatic by the bcc rewriter, other times you'll need to be explicit.

Traceback (most recent call last):
  File "./tcptracer.py.2", line 628, in <module>
    b.attach_kretprobe(event="tcp_v4_connect", fn_name="trace_connect_v4_return")
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 552, in attach_kretprobe
    fn = self.load_func(fn_name, BPF.KPROBE)
  File "/usr/lib/python2.7/dist-packages/bcc/__init__.py", line 346, in load_func
    (func_name, errstr))
Exception: Failed to load BPF program trace_connect_v4_return: Permission denied

It does work on 4.4.0-98-generic. Maybe I should find another 4.9 to try.

[alexgpg]

It works on 4.4.0-103-generic Ubuntu 16.04 Xenial.

[yonghong-song]

The error above seems unrelated:
b.attach_kretprobe(event="tcp_v4_connect", fn_name="trace_connect_v4_return")
has failures, while the change is in function trace_accept_return for kprobe:
b.attach_kretprobe(event="inet_csk_accept", fn_name="trace_accept_return")