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: c004a6fe ('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.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/xen-blkfront.c

@@ -576,7 +576,7 @@ struct setup_rw_req {
 	struct blkif_request *ring_req;
 	grant_ref_t gref_head;
 	unsigned int id;
-	/* Only used when persistent grant is used and it's a read request */
+	/* Only used when persistent grant is used and it's a write request */
 	bool need_copy;
 	unsigned int bvec_off;
 	char *bvec_data;