l2tp: hold socket before dropping lock in l2tp_ip{, 6}_recv()

l2tp: hold socket before dropping lock in l2tp_ip{, 6}_recv()

commit a3c18422a4b4e108bcf6a2328f48867e1003fd95 upstream.

Socket must be held while under the protection of the l2tp lock; there
is no guarantee that sk remains valid after the read_unlock_bh() call.

Same issue for l2tp_ip and l2tp_ip6.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2:
 - Drop changes in l2tp_ip6.c
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>