contrib: dracut: zfs-lib: simplify ask_for_password

The only user is mount-zfs.sh (non-systemd systems),
so reduce it to what it needs

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13291