rst: unmap restore memory after seccomp restore

In order to restore seccomp filters, we need to have access to dynamically
allocated memory from the restorer blob, so we should unmap this memory
afterwards. In order to do this, we need to suspend seccomp earlier, right
after we attach to the tasks instead of just before we do the unmap of the
restorer blob itself.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>