NewNamed(), DeleteNamed() - Missing a mutex

[davidhadas]

NewNamed(), DeleteNamed() missing support multithreaded environments.

A mutex need to be added to ensure that when a file is created it is also mounted without an attempt from a different threat to use, create or delete the same netns.

A mutex need to be added to ensure that when a file is unmounted it is also deleted without an attempt from a different threat to use, create or delete the same netns.

[jeffwidman]

A PR would be most welcome.