Debian has a panic() function which makes it possible to disable shell
access in initramfs by setting the panic kernel parameter. Use it.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kash Pande <kash@tripleback.net>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes#8448