fix(zfs): Install ZFS only when needed (#26)

In situations where this script runs on alternative Arch Linux live
CD ISOs such as github.com/stevleibelt/arch-linux-live-cd-iso-with
-zfs we may not have to insall ZFS kernel modules. Test if the 'zfs'
module is loaded and skip installation if yes.

setup.sh

@@ -451,7 +451,12 @@ function install_pkgs () {
 function install_zfs () {
     #1.6
     declare reset_colors='\033[0m'
+    if modinfo 'zfs' &>/dev/null; then
+        >&3 printf -- '%s\n' \
+            'ZFS kernel module is loaded, no need to install ...'
+    else
         curl -s 'https://raw.githubusercontent.com/eoli3n/archiso-zfs/master/init' | bash
+    fi
     printf -- "${reset_colors}"
 }