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

contrib/dracut/90zfs/zfs-lib.sh.in

@@ -144,7 +144,7 @@ ask_for_password() {
 
     { flock -s 9;
         # Prompt for password with plymouth, if installed and running.
-        if whereis plymouth >/dev/null 2>&1 && plymouth --ping 2>/dev/null; then
+        if type plymouth >/dev/null 2>&1 && plymouth --ping 2>/dev/null; then
             plymouth ask-for-password \
                 --prompt "$ply_prompt" --number-of-tries="$ply_tries" \
                 --command="$ply_cmd"