crypto: arm64/aes-neonbs-ctr - fallback to plain NEON for final chunk

Instead of processing the entire input with the 8-way bit sliced
algorithm, which is sub-optimal for inputs that are not a multiple of
128 bytes in size, invoke the plain NEON version of CTR for the
remainder of the input after processing the bulk using 128 byte strides.

This allows us to greatly simplify the asm code that implements CTR, and
get rid of all the branches and special code paths. It also gains us a
couple of percent of performance.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>