pppoe: Fix sending of PADT on connection termination (#470)

Commit 2b4166d02ed0 ("Close discovery socket after session completed",
2020-11-26) arranged for the discovery socket to be closed when the
PPPOE session negotiation was complete.  However, the discovery socket
is used for sending a PADT message when the connection terminates, and
now that doesn't work because the socket has been closed.

To fix this, we reopen the discovery socket in order to send the PADT
message.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>