Add locking to xattrs Get() and Remove()

[dragonchaser]

In case of other processes having a Write lock that needs to be respected.

refs https://github.com/owncloud/ocis/issues/4251

[update-docs[bot]]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[dragonchaser]

This PR does not resolve the issue reported in https://github.com/owncloud/ocis/issues/4251 reverting to draft for now