daemon.cleanupMounts(): use mount.SingleEntryFilter

Use mount.SingleEntryFilter as we're only interested in a single entry.

Test case data of TestShouldUnmountRoot is modified accordingly, as
from now on:

1. `info` can't be nil;

2. the mountpoint check is not performed (as SingleEntryFilter
   guarantees it to be equal to daemon.root).

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

daemon/daemon_linux_test.go

@@ -179,12 +179,6 @@ func TestShouldUnmountRoot(t *testing.T) {
 			info:   &mount.Info{Root: "/docker", Mountpoint: "/docker"},
 			expect: true,
 		},
-		{
-			desc:   "not a mountpoint",
-			root:   "/docker",
-			info:   nil,
-			expect: false,
-		},
 		{
 			desc:   "root is at in a submount from `/`",
 			root:   "/foo/docker",
@@ -197,12 +191,6 @@ func TestShouldUnmountRoot(t *testing.T) {
 			info:   &mount.Info{Root: "/docker/volumes/1234657/_data", Mountpoint: "/docker"},
 			expect: false,
 		},
-		{
-			desc:   "root is mounted in from a parent mount namespace different root dir",
-			root:   "/foo/bar",
-			info:   &mount.Info{Root: "/docker/volumes/1234657/_data", Mountpoint: "/foo/bar"},
-			expect: false,
-		},
 	} {
 		t.Run(test.desc, func(t *testing.T) {
 			for _, options := range []struct {