sniff fails when the iface parameter is a list

[aliawa]

Please see next comment for bug description

[aliawa]

sniff fails when the iface parameter is a list e.g. iface=["eth1", "eth2"]

Environment:
Scapy version: 2.4.5
Python version: 3.6.9
Operating System: Ubuntu 18.04.5 LTS

How to reproduce:
start sniff like sniff(iface=["eth1","eth2"], prn=lambda x: x.summary())

Actual result

>>>  sniff(iface=["eth1","eth2"], prn=lambda x:x.summary())
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/user1/ali-pan-154892/my_python3/lib/python3.6/site-packages/scapy/sendrecv.py", line 1263, in sniff
    sniffer._run(*args, **kwargs)
  File "/home/user1/ali-pan-154892/my_python3/lib/python3.6/site-packages/scapy/sendrecv.py", line 1128, in _run
    **karg)] = iface
  File "/home/user1/ali-pan-154892/my_python3/lib/python3.6/site-packages/scapy/arch/linux.py", line 501, in __init__
    set_promisc(self.ins, self.iface)
  File "/home/user1/ali-pan-154892/my_python3/lib/python3.6/site-packages/scapy/arch/linux.py", line 181, in set_promisc
    mreq = struct.pack("IHH8s", get_if_index(iff), PACKET_MR_PROMISC, 0, b"")
  File "/home/user1/ali-pan-154892/my_python3/lib/python3.6/site-packages/scapy/arch/linux.py", line 401, in get_if_index
    return int(struct.unpack("I", get_if(iff, SIOCGIFINDEX)[16:20])[0])
  File "/home/user1/ali-pan-154892/my_python3/lib/python3.6/site-packages/scapy/arch/common.py", line 65, in get_if
    return ioctl(sck, cmd, struct.pack("16s16x", iff.encode("utf8")))
AttributeError: 'list' object has no attribute 'encode'

Expected result:
it should start sniffer on eth1 and eth2

[guedou]

It is a duplicate of #3191. Please reopen the issue if it is not correct