lib/raid6: replace custom zero page with ZERO_PAGE

Use the system-wide zero page instead of a custom zero page.

[herbert@gondor.apana.org.au: update lib/raid6/recov_rvv.c, per Klara]
  Link: https://lkml.kernel.org/r/aFkUnXWtxcgOTVkw@gondor.apana.org.au
Link: https://lkml.kernel.org/r/Z9flJNkWQICx0PXk@gondor.apana.org.au


Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Song Liu <song@kernel.org>
Cc: Yu Kuai <yukuai3@huawei.com>
Cc: Klara Modin <klarasmodin@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>