FEXRootFSFetcher: Update and fix xxhash file hashing

The final tail of the file reading was incorrect, so our hashing was
"correct" but it was using stale data from the previous block size read.

Noticed this while wiring up the CI rootfs fetching since the hashing is
a lot simpler there.

Now instead of reading a tail, just attempt to read the full block size
and use the resulting data size instead. Confirmed it matches expected
results now.

In the process we are going to need to update hyperlinks and hashes
anyway, change the hash to XXH3 so it is faster to run.

Scripts/InstallFEX.py

@@ -306,12 +306,12 @@ def GetRootFSPath():
     return _RootFSPath
 
 def CheckRootFSInstallStatus():
-    # Matches what is available on https://rootfs.fex-emu.org/file/fex-rootfs/RootFS_links.txt
+    # Matches what is available on https://rootfs.fex-emu.org/file/fex-rootfs/RootFS_links_XXH3.txt
     UbuntuVersionToRootFS = {
         "20.04": "Ubuntu_21_04.sqsh",
         "21.04": "Ubuntu_21_04.sqsh",
         "21.10": "Ubuntu_21_10.sqsh",
-        "22.04": "Ubuntu_21_10.sqsh",
+        "22.04": "Ubuntu_22_04.sqsh",
     }
 
     return os.path.exists(GetRootFSPath() + UbuntuVersionToRootFS[GetDistro()[1]])