BACKPORT: l2tp: remove ->recv_payload_hook

The tunnel reception hook is only used by l2tp_ppp for skipping PPP
framing bytes. This is a session specific operation, but once a PPP
session sets ->recv_payload_hook on its tunnel, all frames received by
the tunnel will enter pppol2tp_recv_payload_hook(), including those
targeted at Ethernet sessions (an L2TPv3 tunnel can multiplex PPP and
Ethernet sessions).

So this mechanism is wrong, and uselessly complex. Let's just move this
functionality to the pppol2tp rx handler and drop ->recv_payload_hook.

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

(cherry picked from commit 2b139e6b1ec86e1d3646039965dd79ad05d8dca4)
Bug: 116424816
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Change-Id: I581776021730a0a606535118fc3d5ea8540b8e8d