Only alter fstab if needed

src/freedombone-utils-filesystem

@@ -124,7 +124,11 @@ function defrag_filesystem {
 }
 
 function optimise_filesystem {
-    sed -i 's|btrfs subvol=@|btrfs defaults,subvol=@,compress=lzo,ssd|g' /etc/fstab
+    if grep -q "btrfs" /etc/fstab; then
+        if ! grep -q "btrfs defaults,subvol=@,compress=lzo,ssd" /etc/fstab; then
+            sed -i 's|btrfs subvol=@|btrfs defaults,subvol=@,compress=lzo,ssd|g' /etc/fstab
+        fi
+    fi
 }
 
 # NOTE: deliberately no exit 0