Merge branch 'ipv6-avoid-atomic-fragment-on-gso-output'

Yan Zhai says:

====================
ipv6: avoid atomic fragment on GSO output

When the ipv6 stack output a GSO packet, if its gso_size is larger than
dst MTU, then all segments would be fragmented. However, it is possible
for a GSO packet to have a trailing segment with smaller actual size
than both gso_size as well as the MTU, which leads to an "atomic
fragment". Atomic fragments are considered harmful in RFC-8021. An
Existing report from APNIC also shows that atomic fragments are more
likely to be dropped even it is equivalent to a no-op [1].

The series contains following changes:
* drop feature RTAX_FEATURE_ALLFRAG, which has been broken. This helps
  simplifying other changes in this set.
* refactor __ip6_finish_output code to separate GSO and non-GSO packet
  processing, mirroring IPv4 side logic.
* avoid generating atomic fragment on GSO packets.

Link: https://www.potaroo.net/presentations/2022-03-01-ipv6-frag.pdf
V4: https://lore.kernel.org/netdev/cover.1698114636.git.yan@cloudflare.com/
V3: https://lore.kernel.org/netdev/cover.1697779681.git.yan@cloudflare.com/
V2: https://lore.kernel.org/netdev/ZS1%2Fqtr0dZJ35VII@debian.debian/
====================

Link: https://lore.kernel.org/r/cover.1698156966.git.yan@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>