Using 3 blocks here doesn't give us much more than using 2, and it
causes a stack frame size warning on certain compiler/config/arch
combinations:

   lib/crypto/blake2s-selftest.c: In function 'blake2s_selftest':
>> lib/crypto/blake2s-selftest.c:632:1: warning: the frame size of 1088 bytes is larger than 1024 bytes [-Wframe-larger-than=]
     632 | }
         | ^

So this patch just reduces the block from 3 to 2, which makes the
warning go away.
Reported-by: Nkernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/linux-crypto/202206200851.gE3MHCgd-lkp@intel.com
Fixes: 2d16803c ("crypto: blake2s - remove shash module")
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>