Close discovery socket after session completed

After the session is complete, the socket is left unmanaged. When the
interface receives PADIs from other device, the packets is putting
in the socket's Recv-Q, which eat system memory.

[root@test ~]# ss -f link
Netid  Recv-Q Send-Q    Local Address:Port    Peer Address:Port
p_raw  10269952 0          ppp_disc:eth1           *

Signed-off-by: Xing Qingjie <88930741@qq.com>