ppp: don't set sk_state to PPPOX_ZOMBIE in pppoe_disc_rcv()

Since 287f3a94 ("pppoe: Use workqueue to die properly when a PADT
is received"), pppoe_disc_rcv() disconnects the socket by scheduling
pppoe_unbind_sock_work(). This is enough to stop socket transmission
and makes the PPPOX_ZOMBIE state uncessary.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>