pkg/mount: refactor Unmount()

It has been pointed out that we're ignoring EINVAL from umount(2)
everywhere, so let's move it to a lower-level function. Also, its
implementation should be the same for any UNIX incarnation, so
let's consolidate it.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

pkg/mount/unmount_unix.go

@@ -0,0 +1,17 @@
+// +build !windows
+
+package mount // import "github.com/docker/docker/pkg/mount"
+
+import "golang.org/x/sys/unix"
+
+func unmount(target string, flags int) error {
+	err := unix.Unmount(target, flags)
+	if err == unix.EINVAL {
+		// Ignore "not mounted" error here. Note the same error
+		// can be returned if flags are invalid, so this code
+		// assumes that the flags value is always correct.
+		err = nil
+	}
+
+	return err
+}