fix(ip6tc): ipv6 interface name with 15 chars is too big

[oryaacov]

File "/home/zn-admin/.zn-internal/venv3/lib/python3.8/site-packages/iptc/easy.py", line 303, in encode_iptc_rule
    setattr(iptc_rule, name.replace('-', '_'), rule_d[name])
  File "/home/zn-admin/.zn-internal/venv3/lib/python3.8/site-packages/iptc/ip6tc.py", line 486, in set_out_interface
    raise ValueError("interface name %s too long" % (intf))
ValueError: interface name br-13786b5c3131\x00  too long

IPv4 and IPv6 code for getting in/out interface name is different, while IPv6 contained the following bug if the interface name is 15 characters:
for i in range(_IFNAMSIZ): will never run the else section which deletes the last null character using intf = intf[:-1]if the interface name is 15 chars.

I aligned the logic to use the same one as ipv4, since both function are doing the same thing, just in a different way

before the fix:

after the fix: