udev: add helper udev_device_new_from_stat_rdev()

This is a simple wrapper around udev_device_new_from_devnum(), and uses
the data from a struct stat's .st_rdev field to derive the udev_device
object.

src/shared/udev-util.c

@@ -42,3 +42,26 @@ int udev_parse_config(void) {
 
         return 0;
 }
+
+int udev_device_new_from_stat_rdev(struct udev *udev, const struct stat *st, struct udev_device **ret) {
+        struct udev_device *nd;
+        char type;
+
+        assert(udev);
+        assert(st);
+        assert(ret);
+
+        if (S_ISBLK(st->st_mode))
+                type = 'b';
+        else if (S_ISCHR(st->st_mode))
+                type = 'c';
+        else
+                return -ENOTTY;
+
+        nd = udev_device_new_from_devnum(udev, type, st->st_rdev);
+        if (!nd)
+                return -errno;
+
+        *ret = nd;
+        return 0;
+}

src/shared/udev-util.h

@@ -21,3 +21,5 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(struct udev_ctrl_msg*, udev_ctrl_msg_unref);
 DEFINE_TRIVIAL_CLEANUP_FUNC(struct udev_monitor*, udev_monitor_unref);
 
 int udev_parse_config(void);
+
+int udev_device_new_from_stat_rdev(struct udev *udev, const struct stat *st, struct udev_device **ret);