zfs: Use unix.Open to verify that /dev/zfs exists

The alternative os.OpenFile internally tries to enable non-blocking i/o
on the descriptor which on FreeBSD has the side effect of printing a
cryptic error message on the console
(https://github.com/openzfs/zfs/issues/14286).

Signed-off-by: Doug Rabson <dfr@rabson.org>

drivers/zfs/zfs.go

@@ -57,12 +57,12 @@ func Init(base string, opt graphdriver.Options) (graphdriver.Driver, error) {
 		return nil, fmt.Errorf("the 'zfs' command is not available: %w", graphdriver.ErrPrerequisites)
 	}
 
-	file, err := os.OpenFile("/dev/zfs", os.O_RDWR, 0600)
+	file, err := unix.Open("/dev/zfs", unix.O_RDWR, 0600)
 	if err != nil {
 		logger.Debugf("cannot open /dev/zfs: %v", err)
 		return nil, fmt.Errorf("could not open /dev/zfs: %v: %w", err, graphdriver.ErrPrerequisites)
 	}
-	defer file.Close()
+	defer unix.Close(file)
 
 	options, err := parseOptions(opt.DriverOptions)
 	if err != nil {