proc: Set correct userns for new proc super created by a new pid_namespace

Fix the setting up a new proc superblock for a new pid_namespace such that
the user_ns for that proc superblock needs to be taken from the new
pid_namespace and not the active process.

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>