xen/blkfront: improve protection against issuing unsupported REQ_FUA

Guard against issuing unsupported REQ_FUA and REQ_FLUSH was introduced
in d11e6158 and was factored out into blkif_request_flush_valid() in
0f1ca65e. However:
1) This check in incomplete. In case we negotiated to feature_flush = REQ_FLUSH
   and flush_op = BLKIF_OP_FLUSH_DISKCACHE (so FUA is unsupported) FUA request
   will still pass the check.
2) blkif_request_flush_valid() is misnamed. It is bool but returns true when
   the request is invalid.
3) When blkif_request_flush_valid() fails -EIO is being returned. It seems that
   -EOPNOTSUPP is more appropriate here.
Fix all of the above issues.

This patch is based on the original patch by Laszlo Ersek and a comment by
Jeff Moyer.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkfront.c

@@ -582,12 +582,14 @@ static inline void flush_requests(struct blkfront_info *info)
 		notify_remote_via_irq(info->irq);
 }
 
-static inline bool blkif_request_flush_valid(struct request *req,
-					     struct blkfront_info *info)
+static inline bool blkif_request_flush_invalid(struct request *req,
+					       struct blkfront_info *info)
 {
 	return ((req->cmd_type != REQ_TYPE_FS) ||
-		((req->cmd_flags & (REQ_FLUSH | REQ_FUA)) &&
-		!info->flush_op));
+		((req->cmd_flags & REQ_FLUSH) &&
+		 !(info->feature_flush & REQ_FLUSH)) ||
+		((req->cmd_flags & REQ_FUA) &&
+		 !(info->feature_flush & REQ_FUA)));
 }
 
 /*
@@ -612,8 +614,8 @@ static void do_blkif_request(struct request_queue *rq)
 
 		blk_start_request(req);
 
-		if (blkif_request_flush_valid(req, info)) {
-			__blk_end_request_all(req, -EIO);
+		if (blkif_request_flush_invalid(req, info)) {
+			__blk_end_request_all(req, -EOPNOTSUPP);
 			continue;
 		}