CVE-2026-98611: bpf: Check params size before reading reserved fields

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

bpf: Check params size before reading reserved fields

bpfcryptoctxcreate() is a kfunc whose second argument is declared with the sz annotation, so the verifier only guarantees that paramssz bytes of params are valid. The function nevertheless reads params->reserved[0] and params->reserved[1] (offsets 14 and 15) before comparing paramssz against the size of struct bpfcryptoparams, so a BPF program can pass a shorter buffer and have the kernel read past the region that was validated for it.

Move the size check in front of the reserved field reads.