namespaces: Simplify copy_namespaces so it is clear what is going on.

Remove the test for the impossible case where tsk->nsproxy == NULL.  Fork
will never be called with tsk->nsproxy == NULL.

Only call get_nsproxy when we don't need to generate a new_nsproxy,
and mark the case where we don't generate a new nsproxy as likely.

Remove the code to drop an unnecessarily acquired nsproxy value.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>