FROMGIT: crypto: arm/chacha20 - limit the preemption-disabled section

To improve responsivesess, disable preemption for each step of the walk
(which is at most PAGE_SIZE) rather than for the entire
encryption/decryption operation.

Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

(cherry picked from commit be2830b15b60011845ad701076511e8b93b2fd76
 https://git.kernel.org/pub/scm/linux/kernel/git/herbert/cryptodev-2.6.git

 master)
Bug: 112008522
Test: As series, see Ic61c13b53facfd2173065be715a7ee5f3af8760b
Change-Id: I21bfa9c14635e695b128c87df53fea505c3cdd4e
Signed-off-by: Eric Biggers <ebiggers@google.com>