services/content: fix reading a blob which is smaller than the read buffer.

The newly added test fails without this fix in services/content/service.go:

    $ go test -c . && sudo ./containerd.test -test.v -test.root -test.run TestContentClient
    ...
        --- FAIL: TestContentClient/SmallBlob (0.02s)
            provideringester.go:62: rpc error: code = OutOfRange desc = read
    past object length 6 bytes
            helpers.go:67: drwx------       4096
    /tmp/content-suite-ContentClient-286788688
    FAIL

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

content/testsuite/testsuite.go

@@ -50,6 +50,10 @@ func ContentSuite(t *testing.T, name string, storeFn func(ctx context.Context, r
 
 	t.Run("CrossNamespaceAppend", makeTest(t, name, storeFn, checkCrossNSAppend))
 	t.Run("CrossNamespaceShare", makeTest(t, name, storeFn, checkCrossNSShare))
+
+	t.Run("SmallBlob", makeTest(t, name, storeFn, func(ctx context.Context, t *testing.T, cs content.Store) {
+		TestSmallBlob(ctx, t, cs)
+	}))
 }
 
 // ContextWrapper is used to decorate new context used inside the test