CVE-2026-72451: xfrm: Fix xfrm state cache insertion race

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

xfrm: Fix xfrm state cache insertion race

The xfrm input state cache insertion code checks the validity of the state before acquiring the global xfrmstatelock. Thus it's possible for someone else to kill the state after it passed the validity check, and then the insertion will add the dead state to the cache.

Fix this by moving the validity check inside the lock.

This entire function is called on the input path, where BH must be off (e.g., the caller of this function xfrminput acquires its spinlocks without disabling BH).

So there is no need to disable BH here or take the RCU read lock. Remove both and replace them with an assertion that trips if BH is accidentally enabled on some future calling path.