lib/raid6: update recov_rvv.c zero page usage

Update lib/raid6/recov_rvv.c, for 1857fcc8 ("lib/raid6: replace custom
zero page with ZERO_PAGE"), per Klara.

Link: https://lkml.kernel.org/r/aFkUnXWtxcgOTVkw@gondor.apana.org.au


Fixes: 1857fcc8 ("lib/raid6: replace custom zero page with ZERO_PAGE")
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>