contrib/initramfs/scripts/zfs: shellcheck fixup

I got a newer shellcheck, and it pointed out that read without a target
variable is not POSIXly. The var was removed in c3ef9f7528, so I put it
back, and now shellcheck complains about an unused var. That's actually
correct, but necessary, so I've added a suppression for that, probably
better.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17626