Add drvfs and lxfs to GetDriveType

[stephentoub]

Fixes https://github.com/dotnet/corefx/issues/11570
cc: @karelz, @weshaggard

[weshaggard]

LGTM - thanks @stephentoub

[weshaggard]

The PropertiesOfValidDrive test still seem to fail while running on my Ubuntu Docker image on windows.

         /d/git/corefx/bin/AnyOS.AnyCPU.Debug/System.IO.FileSystem.DriveInfo.Tests/netstandard/RunTests.sh /d/git/corefx/Tools/testdotnetcli
         Using as the test runtime folder.
         Running tests... Start time: 21:10:08
         Commands:
         /d/git/corefx/Tools/testdotnetcli/dotnet xunit.console.netcore.exe System.IO.FileSystem.DriveInfo.Tests.dll -xml testResults.xml -notrait Benchmark=true -notrait category=nonnetcoreapp1.1tests -notrait category=OuterLoop -notrait category=failing -notrait category=nonlinuxtests
         /d/git/corefx/bin/AnyOS.AnyCPU.Debug/System.IO.FileSystem.DriveInfo.Tests/netstandard /d/git/corefx/src/System.IO.FileSystem.DriveInfo/tests
         xUnit.net console test runner (64-bit .NET Core)
         Copyright (C) 2014 Outercurve Foundation.
         
         Discovering: System.IO.FileSystem.DriveInfo.Tests
         Discovered:  System.IO.FileSystem.DriveInfo.Tests
         Starting:    System.IO.FileSystem.DriveInfo.Tests
            System.IO.FileSystem.DriveInfoTests.DriveInfoUnixTests.PropertiesOfValidDrive [FAIL]
               Assert.Equal() Failure
               Expected: Fixed
               Actual:   Unknown
               Stack Trace:
                  /d/git/corefx/src/System.IO.FileSystem.DriveInfo/tests/DriveInfo.Unix.Tests.cs(69,0): at System.IO.FileSystem.DriveInfoTests.DriveInfoUnixTests.PropertiesOfValidDrive()
         Finished:    System.IO.FileSystem.DriveInfo.Tests

[stephentoub]

Hmm. Wes, can you see what string is being passed into the GetDriveType function:
https://github.com/dotnet/corefx/pull/15451/files#diff-941e76780713c4bffc85a785df882374R129
such that it's falling through to return Unknown:
https://github.com/dotnet/corefx/pull/15451/files#diff-941e76780713c4bffc85a785df882374R279
?

[weshaggard]

null.

Here is what we get back from the call to GetFormatInfoFromMountPoint:
NumericFormat=2035054128, name=/, formatBuffer=0, format=null, type=Unknown

We get a buffer of with all zero's which Marshal.PtrToStringAnsi returns null for.

[stephentoub]

Thanks, Wes. Looks like that value corresponds to overlayfs, which is a file system that effectively wraps other file systems. The good news is it's easy for us to add to the table. The bad news is we don't really know what kind of file system it is, since it's wrapping one or more others, so we'll need to weaken the test.