lib/crypto: tests: Fix KMSAN warning in test_sha256_finup_2x()

lib/crypto: tests: Fix KMSAN warning in test_sha256_finup_2x()

Fully initialize *ctx, including the buf field which sha256_init()
doesn't initialize, to avoid a KMSAN warning when comparing *ctx to
orig_ctx.  This KMSAN warning slipped in while KMSAN was not working
reliably due to a stackdepot bug, which has now been fixed.

Fixes: 6733968be7cb ("lib/crypto: tests: Add tests and benchmark for sha256_finup_2x()")
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20251121033431.34406-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>