Added LAG member check on addLagMember() (#2464)

[yenlu-keith]

What I did
Added a check into addLagMember() whether this new LAG member still exists in the kernel.

Why I did it
During syncd container autorestart scenario, on syncd exit, the host interfaces (tun/tap netdevs) go to the DOWN state and then get removed.

Due to the validation as follows, the teammgr will receive the notification about the port state change (the information will be updated in the state DB and pubsub message sent) but the port state record will not be removed from the state DB on port delete:

sonic-swss/portsyncd/linksync.cpp
Line 210 in 7cc035f
if (master && nlmsg_type == RTM_DELLINK)
Due to this, on port state change notification, the isPortStateOk() will succeed and TeamMgr::addLagMember() will be executed even the host interface was actually removed.

The operation is expected to be ignored if the port is already enslaved:

sonic-swss/cfgmgr/teammgr.cpp
Line 721 in 7cc035f
if (isPortEnslaved(member))
The check fails since the port has already been removed:

sonic-swss/cfgmgr/teammgr.cpp
Line 412 in 7cc035f
return lstat(path.c_str(), &buf) == 0;
As a result, the TeamMgr::addLagMember() logic will be executed and failed:

Jun 21 11:47:12.265955 cab18-7-dut INFO teamd#/supervisord: teammgrd Cannot find device "Ethernet0"
Jun 21 11:47:12.294550 cab18-7-dut INFO teamd#/supervisord: teammgrd libteamdctl: cli_usock_process_msg: usock: Error message received: "NoSuchDev"
Jun 21 11:47:12.294550 cab18-7-dut INFO teamd#/supervisord: teammgrd libteamdctl: cli_usock_process_msg: usock: Error message content: "No such device."
Jun 21 11:47:12.294550 cab18-7-dut INFO teamd#/supervisord: teammgrd command call failed (Invalid argument)
Jun 21 11:47:12.322497 cab18-7-dut INFO teamd#/supervisord: teammgrd libteamdctl: cli_usock_process_msg: usock: Error message received: "NoSuchDev"
Jun 21 11:47:12.322497 cab18-7-dut INFO teamd#/supervisord: teammgrd libteamdctl: cli_usock_process_msg: usock: Error message content: "No such device."
Jun 21 11:47:12.322497 cab18-7-dut INFO teamd#/supervisord: teammgrd command call failed (Invalid argument)
Jun 21 11:47:12.328844 cab18-7-dut ERR teamd#teammgrd: :- checkPortIffUp: Failed to get port Ethernet0 flags
Jun 21 11:47:12.328844 cab18-7-dut ERR teamd#teammgrd: :- addLagMember: Failed to add Ethernet0 to port channel PortChannel102
The issue started to reproduce after #2233

How I verified it

autorestart/test_container_autorestart.py -k 'syncd'

[prsunny]

Cherry-picking PR #2464

[prsunny]

@yenlu-keith , i've added labels to original PR. Branch owner shall get back if there are any conflicts and require manual PR. Until then we can hold off on this PR.

[prsunny]

@yenlu-keith , closing this PR as it is already included as part of cherry-picking original PR. Please check the labels