Fs: Fix UNC remapping code.

For a Win32 path such as C:\foo we rewrite it to \\?\C:\foo. For a network share \\foo\bar this needs to become \\?\UNC\foo\bar but we were accidentally making it \\?\UNC\\foo\bar notice the double \\. That's an invalid path.

Change tested manually using a local path accessed through \\localhost\c$\<path>

Fixes #17788 (closed)

@bgamari are you still using the binary patcher for the 8.10 release? If so I'll update it with the fix tonight.