CVE-2026-31641: rxrpc: Fix RxGK token loading to check bounds

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

rxrpc: Fix RxGK token loading to check bounds

rxrpcpreparsexdryfsrxgk() reads the raw key length and ticket length from the XDR token as u32 values and passes each through roundup(x, 4) before using the rounded value for validation and allocation. When the raw length is >= 0xfffffffd, roundup() wraps to 0, so the bounds check and kzalloc both use 0 while the subsequent memcpy still copies the original ~4 GiB value, producing a heap buffer overflow reachable from an unprivileged addkey() call.

Fix this by:

(1) Rejecting raw key lengths above AFSTOKENGKKEYMAX and raw ticket lengths above AFSTOKENGKTOKENMAX before rounding, consistent with the caps that the RxKAD path already enforces via AFSTOKENRKTIXMAX.

(2) Sizing the flexible-array allocation from the validated raw key length via structsizet() instead of the rounded value.

(3) Caching the raw lengths so that the later field assignments and memcpy calls do not re-read from the token, eliminating a class of TOCTOU re-parse.

The control path (valid token with lengths within bounds) is unaffected.