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

rxrpc_preparse_xdr_yfs_rxgk() reads the raw key length and ticket length
from the XDR token as u32 values and passes each through round_up(x, 4)
before using the rounded value for validation and allocation. When the raw
length is >= 0xfffffffd, round_up() 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 add_key() call.

Fix this by:

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

(2) Sizing the flexible-array allocation from the validated raw key
length via struct_size_t() 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.