linux: make patch invocation idempotent

patch.sh

@@ -181,8 +181,15 @@ rollback () {
 
 patch () {
     patch_common
-    if [[ ! -f "$backup_path/$object.$driver_version" ]]; then
-        echo "Attention! Backup not found. Copy current $object to backup."
+    if [[ -f "$backup_path/$object.$driver_version" ]]; then
+        bkp_hash="$(sha1sum "$backup_path/$object.$driver_version" | cut -f1 -d\ )"
+        drv_hash="$(sha1sum "$driver_dir/$object.$driver_version" | cut -f1 -d\ )"
+        if [[ "$bkp_hash" != "$drv_hash" ]] ; then
+            echo "Backup exists and driver file differ from backup. Skipping patch."
+            return 0
+        fi
+    else
+        echo "Attention! Backup not found. Copying current $object to backup."
         mkdir -p "$backup_path"
         cp -p "$driver_dir/$object.$driver_version" \
            "$backup_path/$object.$driver_version"