pkg/ns: fixed the check for network namespace path.

pkg/ns: fixed the check for network namespace path.

The expectation on older kernels (< 3.19) was to have the network
namespace always be a directory. This is not true if the network
namespace is bind mounted to a file, and will make the plugin fail
erroneously in such cases. The fix is to remove this assumption
completely and just do a basic check on the file system types being
returned.

Fixes #288