kernel: Backport upstream flowtable patches from 5.15

kernel: Backport upstream flowtable patches from 5.15

This backports some patches from kernel 5.15 to fix issues with
flowtable offloading in kernel 5.10. OpenWrt backports most of the
patches related to flowtable offloading from kernel 5.15 already, but we
are missing some of the extra fixes.

This fixes some connection tracking problems when a flow gets removed
from the offload and added to the normal SW path again.

The patch 614-v5.18-netfilter-flowtable-fix-TCP-flow-teardown.patch was
extended manually with the nf_conntrack_tcp_established() function.

All changes are already included in kernel 5.15.

Fixes: #8776
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>