crypto: xts - Fix atomic sleep when walking skcipher

When we perform a walk in the completion function, we need to ensure
that it is atomic.

Reported-by:  <syzbot+6f72c20560060c98b566@syzkaller.appspotmail.com>
Fixes: 78105c7e

 ("crypto: xts - Drop use of auxiliary buffer")
Cc: <stable@vger.kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>