Improve fast path of ManagedFileSystem.GetValidFilename

|                      Method |          Data |        Mean |     Error |    StdDev |      Median |  Gen 0 | Gen 1 | Gen 2 | Allocated |
|---------------------------- |-------------- |------------:|----------:|----------:|------------:|-------:|------:|------:|----------:|
|       GetValidFilenameBench |    AC/DCKD/A |    52.29 ns |  0.537 ns |  0.448 ns |    52.35 ns | 0.0255 |     - |     - |      80 B |
|    GetValidFilenameOldBench |    AC/DCKD/A |    86.68 ns |  1.205 ns |  1.127 ns |    86.33 ns | 0.0587 |     - |     - |     184 B |
|    GetValidFilenameWinBench |    AC/DCKD/A |   448.55 ns |  1.228 ns |  1.088 ns |   448.33 ns | 0.0505 |     - |     - |     160 B |
| GetValidFilenameOldWinBench |    AC/DCKD/A |   865.21 ns |  5.734 ns |  5.083 ns |   866.60 ns | 0.0839 |     - |     - |     264 B |
|       GetValidFilenameBench | ValidFileName |    16.00 ns |  0.234 ns |  0.207 ns |    16.02 ns | 0.0102 |     - |     - |      32 B |
|    GetValidFilenameOldBench | ValidFileName |   100.66 ns |  1.255 ns |  1.174 ns |   101.21 ns | 0.0587 |     - |     - |     184 B |
|    GetValidFilenameWinBench | ValidFileName |   116.60 ns |  1.624 ns |  1.519 ns |   116.88 ns | 0.0356 |     - |     - |     112 B |
| GetValidFilenameOldWinBench | ValidFileName | 1,052.66 ns | 18.077 ns | 33.056 ns | 1,037.25 ns | 0.0839 |     - |     - |     264 B |

Emby.Server.Implementations/IO/ManagedFileSystem.cs

@@ -302,28 +302,27 @@ namespace Emby.Server.Implementations.IO
         /// <exception cref="ArgumentNullException">The filename is null.</exception>
         public string GetValidFilename(string filename)
         {
+            var invalid = Path.GetInvalidFileNameChars();
+            var first = filename.IndexOfAny(invalid);
+            if (first == -1)
+            {
+                // Fast path for clean strings
+                return filename;
+            }
+
             return string.Create(
                 filename.Length,
-                filename,
+                (filename, invalid, first),
                 (chars, state) =>
                 {
-                    state.AsSpan().CopyTo(chars);
-
-                    var invalid = Path.GetInvalidFileNameChars();
-
-                    var first = state.AsSpan().IndexOfAny(invalid);
-                    if (first == -1)
-                    {
-                        // Fast path for clean strings
-                        return;
-                    }
+                    state.filename.AsSpan().CopyTo(chars);
 
-                    chars[first++] = ' ';
+                    chars[state.first++] = ' ';
 
                     var len = chars.Length;
-                    foreach (var c in invalid)
+                    foreach (var c in state.invalid)
                     {
-                        for (int i = first; i < len; i++)
+                        for (int i = state.first; i < len; i++)
                         {
                             if (chars[i] == c)
                             {