hooks: drop namespace references before post-stop

hooks: drop namespace references before post-stop

Callers such as LXD run post-stop hooks to perform cleanup operations on
shutdown. This can involve umount and other things. To avoid surprises with
lingering namespace references we should close all our namespace-preserving
file descriptors. We don't need them at this point anymore anyway.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>