namespace: make whole namespace_setup() work regardless of configured umask

namespace: make whole namespace_setup() work regardless of configured umask

Let's reset the umask during the whole namespace_setup() logic, so that
all our mkdir() + mknod() are not subjected to whatever umask might
currently be set.

This mostly moves the umask save/restore logic out of
mount_private_dev() and into the stack frame of namespace_setup() that
is further out.

Fixes #19899

(cherry picked from commit cdf42f9bd40ff21a67d58b948efea055d56ad398)