selftests/net/ipsec: Fix variable size type not at the end of struct

selftests/net/ipsec: Fix variable size type not at the end of struct

The "struct alg" object contains a union of 3 xfrm structures:

	union {
		struct xfrm_algo;
		struct xfrm_algo_aead;
		struct xfrm_algo_auth;
	}

All of them end with a flexible array member used to store key material,
but the flexible array appears at *different offsets* in each struct.
bcz of this, union itself is of variable-sized & Placing it above
char buf[...] triggers:

ipsec.c:835:5: warning: field 'u' with variable sized type 'union
(unnamed union at ipsec.c:831:3)' not at the end of a struct or class
is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
  835 |                 } u;
      |                   ^

one fix is to use "TRAILING_OVERLAP()" which works with one flexible
array member only.

But In "struct alg" flexible array member exists in all union members,
but not at the same offset, so TRAILING_OVERLAP cannot be applied.

so the fix is to explicitly overlay the key buffer at the correct offset
for the largest union member (xfrm_algo_auth). This ensures that the
flexible-array region and the fixed buffer line up.

No functional change.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Link: https://patch.msgid.link/20260109152201.15668-1-ankitkhushwaha.linux@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>