Change DefaultMountVersion to 9p2000.L

[kron4eg]

9p2000.u is a legacy protocol version. This upgrades us to the most recent version with better permissions handling. From the Linux kernel docs: https://landley.net/kdocs/Documentation/filesystems/9p.txt

Around 2003 Plan 9 was open sourced, which led to the development of an
extended version of the protocol, "9p2000.U" (for Unix) providing full posix
semantics .... The current protocol version, "9p2000.L" (for Linux) was developed for v9fs
after a few years of real world experience. It tailors the protocol to Linux
with support for ACLs, file locking, more efficient directory listing, corner
cases in things like file deletion, and so on. 

Fixes #2290

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[minikube-bot]

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[tstromberg]

@minikube-bot OK to test

[tstromberg]

I think this PR is a great idea. Thanks for proposing it! Will approve once I can see that the integration tests pass.

[afbjorklund]

Looks OK, but I think we need to also upgrade the server for it to be of most use ?

[kron4eg]

@afbjorklund I'm not sure I got you. Do you mean third_party/go9p library?

[tstromberg]

@minikube-bot OK to test

FWIW, I think updating the server is unrelated to this version change. L has been around for a very long time.

[afbjorklund]

@kron4eg yes, I think that is serving ?

[afbjorklund]

Afaik, it will negotiate. So we should start with updating this client, but also update server

[kron4eg]

Unfortunately I can't find from where third_party/go9p originated. But it worked with old minikube releases (oldest tested is from 2 years ago) when I've explicitly specified --9p-version=9p2000.L