l2tp: fix reading optional fields of L2TPv3

l2tp: fix reading optional fields of L2TPv3

Use pskb_may_pull() to make sure the optional fields are in skb linear
parts, so we can safely read them later.

It's easy to reproduce the issue with a net driver that supports paged
skb data. Just create a L2TPv3 over IP tunnel and then generates some
network traffic.
Once reproduced, rx err in /sys/kernel/debug/l2tp/tunnels will increase.

Changes in v4:
1. s/l2tp_v3_pull_opt/l2tp_v3_ensure_opt_in_linear/
2. s/tunnel->version != L2TP_HDR_VER_2/tunnel->version == L2TP_HDR_VER_3/
3. Add 'Fixes' in commit messages.

Changes in v3:
1. To keep consistency, move the code out of l2tp_recv_common.
2. Use "net" instead of "net-next", since this is a bug fix.

Changes in v2:
1. Only fix L2TPv3 to make code simple.
   To fix both L2TPv3 and L2TPv2, we'd better refactor l2tp_recv_common.
   It's complicated to do so.
2. Reloading pointers after pskb_may_pull

Fixes: f7faffa3ff8e ("l2tp: Add L2TPv3 protocol support")
Fixes: 0d76751fad77 ("l2tp: Add L2TPv3 IP encapsulation (no UDP) support")
Fixes: a32e0eec7042 ("l2tp: introduce L2TPv3 IP encapsulation support for IPv6")
Signed-off-by: Jacob Wen <jian.w.wen@oracle.com>
Acked-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 4522a70db7aa5e77526a4079628578599821b193)
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	net/l2tp/l2tp_core.c
	net/l2tp/l2tp_ip.c
	net/l2tp/l2tp_ip6.c
Commit 2b139e6b1ec8 ("l2tp: remove ->recv_payload_hook") is not in UEK5.

l2tp_core.h:
    s/l2tp_get_l2specific_len(session)/session->l2specific_len/ due to
    62e7b6a57c7b ("l2tp: remove l2specific_len dependency in l2tp_core")
    is not in UEK4.

Orabug: 29368048

Signed-off-by: Jacob Wen <jian.w.wen@oracle.com>
Reviewed-by: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>