os.Unmount: do not consult mountinfo, drop flags

1. Currently, Unmount() call takes a burden to parse the whole nine yards
of /proc/self/mountinfo to figure out whether the given mount point is
mounted or not (and returns an error in case parsing fails somehow).

Instead, let's just call umount() and ignore EINVAL, which results
in the same behavior, but much better performance.

This also introduces a slight change: in case target does not exist,
the appropriate error (ENOENT) is returned -- document that.

2. As Unmount() is always used with MNT_DETACH flag, let's drop the
flags argument. This way, the only reason of EINVAL returned from
umount(2) can only be "target is not mounted".

3. While at it, remove the 'containerdmount' alias from the package.

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

pkg/os/testing/fake_os.go

@@ -49,7 +49,7 @@ type FakeOS struct {
 	CopyFileFn             func(string, string, os.FileMode) error
 	WriteFileFn            func(string, []byte, os.FileMode) error
 	MountFn                func(source string, target string, fstype string, flags uintptr, data string) error
-	UnmountFn              func(target string, flags int) error
+	UnmountFn              func(target string) error
 	LookupMountFn          func(path string) (containerdmount.Info, error)
 	calls                  []CalledDetail
 	errors                 map[string]error
@@ -230,14 +230,14 @@ func (f *FakeOS) Mount(source string, target string, fstype string, flags uintpt
 }
 
 // Unmount is a fake call that invokes UnmountFn or just return nil.
-func (f *FakeOS) Unmount(target string, flags int) error {
-	f.appendCalls("Unmount", target, flags)
+func (f *FakeOS) Unmount(target string) error {
+	f.appendCalls("Unmount", target)
 	if err := f.getError("Unmount"); err != nil {
 		return err
 	}
 
 	if f.UnmountFn != nil {
-		return f.UnmountFn(target, flags)
+		return f.UnmountFn(target)
 	}
 	return nil
 }