Replace whereis with type in zfs-lib.sh

The whereis command should not be used since it may not exist
in the initramfs. The dracut plymouth module also uses the type
command instead of whereis.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Garrett Fields <ghfields@gmail.com>
Signed-off-by: Dacian Reece-Stremtan <dacianstremtan@gmail.com>
Closes #8920
Closes #8938