linkbox: fix working with names longer than 8-25 Unicode chars.

The LinkBox API does not allow searching by more than 25 Unicode
characters in the name, for this reason it is currently impossible to
work with files and folders named longer than 8 Unicode chars (if
encoded in base32).

This fix queries all files in a directory for long names and checks
their names one by one, thus solving the issue.

Fixes #7542

backend/linkbox/linkbox_test.go

@@ -13,5 +13,7 @@ func TestIntegration(t *testing.T) {
 	fstests.Run(t, &fstests.Opt{
 		RemoteName: "TestLinkbox:",
 		NilObject:  (*linkbox.Object)(nil),
+		// Linkbox doesn't support leading dots for files
+		SkipLeadingDot: true,
 	})
 }