pppoe: delete session hash entries by pointer match

pppoe_connect() copies the requested PPPoE address to a socket before
attempting to insert the socket into the session hash table. If the
tuple already exists, __set_item() returns -EALREADY, but the copied
address and ifindex remain set on the failed socket.

Closing that socket then deletes the existing socket's hash entry
through pppoe_release(), preventing it from receiving further session
traffic. An unprivileged process in the same network namespace can
trigger this through a duplicate connection attempt.

To avoid the accidental delete, pass the pppox_sock to __delete_item(),
and unlink only an entry whose pointer matches the requested socket.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
Link: https://patch.msgid.link/20261020092118.47416-1-qingfang.deng@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>