Shellcheck cleanup for initrd scripts

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Kash Pande <kash@tripleback.net>
Co-authored-by: Matthew Thode <mthode@mthode.org>
Signed-off-by: Kash Pande <kash@tripleback.net>
Signed-off-by: Matthew Thode <mthode@mthode.org>
Closes #7214