pkg/ns: use file system magic numbers from golang.org/x/sys/unix
pkg/ns: use file system magic numbers from golang.org/x/sys/unix

Use the constants already defined in the golang.org/x/sys/unix package
instead of open-coding them.

Signed-off-by: Tobias Klauser <9249c549618448e4c699e8032ba6c7f0d7fd4b7f@distanz.ch>

containernetworking/plugins,containernetworking/plugins