CVE-2026-46193: xfrm: ah: account for ESN high bits in async callbacks

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

xfrm: ah: account for ESN high bits in async callbacks

AH allocates its temporary auth/ICV layout differently when ESN is enabled: the async ahash setup appends a 4-byte seqhi slot before the ICV or authdata area, but the async completion callbacks still reconstruct the temporary layout as if seqhi were absent.

With an async AH implementation selected, that makes AH copy or compare the wrong bytes on both the IPv4 and IPv6 paths. In UML repro on IPv4 AH with ESN and forced async hmac(sha1), ping fails with 100% packet loss, and the callback logs show the pre-fix drift:

ah4 outputdone: esn=1 err=0 icvoff=20 expectedoff=24 ah4 inputdone: esn=1 authoff=20 expectedauthoff=24 icvoff=32 expectedicvoff=36

Reconstruct the callback-side layout the same way the setup path built it by skipping the ESN seqhi slot before locating the saved authdata or ICV. Per RFC 4302, the ESN high-order 32 bits participate in the AH ICV computation, so the async callbacks must account for the seqhi slot.

Post-fix, the same IPv4 AH+ESN+forced-async-hmac(sha1) UML repro shows the corrected offset (ah4 outputdone: esn=1 err=0 icvoff=24 expectedoff=24) and ping succeeds; net/ipv4/ah4.o and net/ipv6/ah6.o build clean at W=1. IPv6 AH+ESN was not exercised at runtime, and the change has not been tested against a real async hardware AH engine.