Add add_prop_list_default helper

Adding to a property list only if there is no existing value is used
twice. Once by zpool create -R and again by zpool import -R. Now that
zpool create -t and zpool import -t also need it, lets refactor it into
a helper function to make the code more readable.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#2417

cmd/zpool/zpool_main.c

@@ -485,6 +485,21 @@ add_prop_list(const char *propname, char *propval, nvlist_t **props,
 	return (0);
 }
 
+/*
+ * Set a default property pair (name, string-value) in a property nvlist
+ */
+static int
+add_prop_list_default(const char *propname, char *propval, nvlist_t **props,
+    boolean_t poolprop)
+{
+	char *pval;
+
+	if (nvlist_lookup_string(*props, propname, &pval) == 0)
+		return (0);
+
+	return (add_prop_list(propname, propval, props, B_TRUE));
+}
+
 /*
  * zpool add [-fn] [-o property=value] <pool> <vdev> ...
  *
@@ -823,11 +838,7 @@ zpool_do_create(int argc, char **argv)
 			if (add_prop_list(zpool_prop_to_name(
 			    ZPOOL_PROP_ALTROOT), optarg, &props, B_TRUE))
 				goto errout;
-			if (nvlist_lookup_string(props,
-			    zpool_prop_to_name(ZPOOL_PROP_CACHEFILE),
-			    &propval) == 0)
-				break;
-			if (add_prop_list(zpool_prop_to_name(
+			if (add_prop_list_default(zpool_prop_to_name(
 			    ZPOOL_PROP_CACHEFILE), "none", &props, B_TRUE))
 				goto errout;
 			break;
@@ -2068,11 +2079,7 @@ zpool_do_import(int argc, char **argv)
 			if (add_prop_list(zpool_prop_to_name(
 			    ZPOOL_PROP_ALTROOT), optarg, &props, B_TRUE))
 				goto error;
-			if (nvlist_lookup_string(props,
-			    zpool_prop_to_name(ZPOOL_PROP_CACHEFILE),
-			    &propval) == 0)
-				break;
-			if (add_prop_list(zpool_prop_to_name(
+			if (add_prop_list_default(zpool_prop_to_name(
 			    ZPOOL_PROP_CACHEFILE), "none", &props, B_TRUE))
 				goto error;
 			break;