keventd: resolve $attr{} and %s{} through the parent chain

Attribute substitution only looked at the device's own sysfs directory,
while the matching key ATTRS{} walks parents. Devices without an
attribute of their own, partitions in particular whose serial lives in
the parent disk, therefore substituted to an empty string.

keventd/sysfs.h

@@ -40,6 +40,15 @@ int sysfs_read_file(const char *path, char *buf, size_t len);
 int sysfs_read_attr(const char *devpath, const char *attr,
 		    char *buf, size_t len);
 
+/*
+ * Read a sysfs attribute for the device at /sys{devpath}, falling back
+ * to the parent chain, partitions have no "serial" of their own, it
+ * lives in the parent disk's sysfs directory.
+ * Returns 0 on success, -1 if no level of the chain has the attribute.
+ */
+int sysfs_read_attr_chain(const char *devpath, const char *attr,
+			  char *buf, size_t len);
+
 /*
  * Walk the sysfs device tree upward from devpath toward /sys/devices,
  * calling cb at each level.  cb receives the absolute syspath and the