Reduce the scope of the kernel_neon_begin/end regions so that the SIMD
unit is released (and thus preemption re-enabled) if the crypto operation
cannot be completed in a single scatterwalk step. This avoids scheduling
blackouts due to preemption being enabled for unbounded periods, resulting
in a more responsive system.

After this change, we can also permit the cipher_walk infrastructure to
sleep, so set the 'atomic' parameter to skcipher_walk_virt() to false as
well.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>