xen/blkfront: fix comment for need_copy

xen/blkfront: fix comment for need_copy

The 'need_copy' is set when rq_data_dir(req) returns WRITE, in order to
copy the written data to persistent page.

".need_copy = rq_data_dir(req) && info->feature_persistent,"

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Fixes: c004a6fe0c40 ('block/xen-blkfront: Make it running on 64KB page granularity')
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220317220930.5698-1-dongli.zhang@oracle.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>