Fix a typo in the code

[wewei]

Problem:

When using the tar-fs with custom fs, the mkdirp method still use the standard fs. The root cause is a typo in mkdirfix method. The file system object is passed with fs option, but consumed through xfs.

Fix:

Replace xfs with fs in mkdirfix.

[wewei]

@mafintosh , thanks for developing tar-fs, it's a great utility package. We need to use the package with custom fs, this is a blocking issue. If the simple fix looks fine, would you help to publish a new patch?

Thanks a lot.

[mafintosh]

Awesome. 1.16.3