zed: Fix config_sync autoexpand flood

Users were seeing floods of `config_sync` events when autoexpand was
enabled.  This happened because all "disk status change" udev events
invoke the autoexpand codepath, which calls zpool_relabel_disk(),
which in turn cause another "disk status change" event to happen,
in a feedback loop.  Note that "disk status change" happens every time
a user calls close() on a block device.

This commit breaks the feedback loop by only allowing an autoexpand
to happen if the disk actually changed size.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes: #7132
Closes: #7366
Closes #13729

include/sys/sysevent/dev.h

@@ -244,6 +244,9 @@ extern "C" {
 #define	DEV_PATH		"path"
 #define	DEV_IS_PART		"is_slice"
 #define	DEV_SIZE		"dev_size"
+
+/* Size of the whole parent block device (if dev is a partition) */
+#define	DEV_PARENT_SIZE		"dev_parent_size"
 #endif /* __linux__ */
 
 #define	EV_V1			1