Merge branch 'l2tp-fix-races-with-ipv4-mapped-ipv6-addresses'

Paolo Abeni says:

====================
l2tp: fix races with ipv4-mapped ipv6 addresses

The syzbot reported an l2tp oops that uncovered some races in the l2tp xmit
path and a partially related issue in the generic ipv6 code.

We need to address them separately.

v1 -> v2:
 - add missing fixes tag in patch 1
 - fix several issues in patch 2

v2 -> v3:
 - dropped some unneeded chunks in patch 2
====================

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