Prevent pointer to an out-of-scope local variable

`show_str` could be a pointer to a local variable in stack
which is out-of-scope by the time
`return (snprintf(buf, buflen, "%s\n", show_str));`
is called.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes openzfs#8924
Closes openzfs#8940

module/zfs/zfs_sysfs.c

@@ -264,6 +264,7 @@ zprop_sysfs_show(const char *attr_name, const zprop_desc_t *property,
     char *buf, size_t buflen)
 {
 	const char *show_str;
+	char number[32];
 
 	/* For dataset properties list the dataset types that apply */
 	if (strcmp(attr_name, "datasets") == 0 &&
@@ -291,8 +292,6 @@ zprop_sysfs_show(const char *attr_name, const zprop_desc_t *property,
 	} else if (strcmp(attr_name, "values") == 0) {
 		show_str = property->pd_values ? property->pd_values : "";
 	} else if (strcmp(attr_name, "default") == 0) {
-		char number[32];
-
 		switch (property->pd_proptype) {
 		case PROP_TYPE_NUMBER:
 			(void) snprintf(number, sizeof (number), "%llu",