crypto: x86/chacha20 - avoid sleeping with preemption disabled

In chacha20-simd, clear the MAY_SLEEP flag in the blkcipher_desc to
prevent sleeping with preemption disabled, under kernel_fpu_begin().

This was fixed upstream incidentally by a large refactoring,
commit 9ae433bc79f9 ("crypto: chacha20 - convert generic and x86
versions to skcipher").  But syzkaller easily trips over this when
running on older kernels, as it's easily reachable via AF_ALG.
Therefore, this patch makes the minimal fix for older kernels.

Fixes: c9320b6d ("crypto: chacha20 - Add a SSSE3 SIMD variant for x86_64")
Cc: linux-crypto@vger.kernel.org
Cc: Martin Willi <martin@strongswan.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>