crypto: tcrypt - fix skcipher multi-buffer tests for 1420B blocks

Commit ad6d66bc ("crypto: tcrypt - include 1420 byte blocks in aead and skcipher benchmarks")
mentions:
> power-of-2 block size. So let's add 1420 bytes explicitly, and round
> it up to the next blocksize multiple of the algo in question if it
> does not support 1420 byte blocks.
but misses updating skcipher multi-buffer tests.

Fix this by using the proper (rounded) input size.

Fixes: ad6d66bc ("crypto: tcrypt - include 1420 byte blocks in aead and skcipher benchmarks")
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>