xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit

In the Linux kernel, the following vulnerability has been resolved:

xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit

The struct pernet_operations docstring in include/net/net_namespace.h

explicitly warns against blocking RCU primitives in .exit handlers:

Exit methods using blocking RCU primitives, such as

synchronize_rcu(), should be implemented via exit_batch.

[...]

Please, avoid synchronize_rcu() at all, where it's possible.

Note that a combination of pre_exit() and exit() can

be used, since a synchronize_rcu() is guaranteed between

the calls.

xfrm_policy_fini() violates this: it calls synchronize_rcu() before

freeing the policy_bydst hash tables (so no RCU reader is mid-

traversal at free time), but runs from xfrm_net_ops.exit -- once per

namespace -- so a cleanup_net() of N namespaces pays N full RCU

grace periods serially.

Use the documented pre_exit/exit split. Move the policy flush (and

the workqueue drains it depends on) into a new .pre_exit handler;

xfrm_policy_fini() then runs in .exit and frees the hash tables

after the synchronize_rcu_expedited() that cleanup_net() guarantees

between the two phases. Providing O(1) RCU grace periods per batch

instead of O(N).

Observed on Linux 6.18 with a workload doing unshare(CLONE_NEWNET)

at ~13/sec sustained: cleanup_net() and the netns_wq rescuer kthread

both stuck in xfrm_policy_fini()'s synchronize_rcu(), >300k struct

net accumulated in the cleanup queue, Percpu in /proc/meminfo climbed

to 130+ GB on 256-CPU hosts, and memcg OOMs followed. setup_net and

__put_net counts were balanced, ruling out a refcount leak.