move sys.FMountat() into mount package

It's the only location this is used, so might as well move it
into that package. I could not find external users of this utility,
so not adding an alias / deprecation.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

mount/mount_linux.go

@@ -24,7 +24,6 @@ import (
 	"strings"
 	"time"
 
-	"github.com/containerd/containerd/sys"
 	"github.com/pkg/errors"
 	"golang.org/x/sys/unix"
 )
@@ -378,7 +377,7 @@ func mountAt(chdir string, source, target, fstype string, flags uintptr, data st
 	if !fs.IsDir() {
 		return errors.Wrap(errors.Errorf("%s is not dir", chdir), "failed to mountat")
 	}
-	return errors.Wrap(sys.FMountat(f.Fd(), source, target, fstype, flags, data), "failed to mountat")
+	return errors.Wrap(FMountat(f.Fd(), source, target, fstype, flags, data), "failed to mountat")
 }
 
 func (m *Mount) mountWithHelper(helperBinary, typePrefix, target string) error {