md: don't print the same repeated messages about delayed sync operation

This fixes a long-standing bug that caused a flood of messages like:
"md: delaying data-check of md1 until md2 has finished (they share one
or more physical units)"

It can be reproduced like this:
1. Create at least 3 raid1 arrays on a pair of disks, each on different
   partitions.
2. Request a sync operation like 'check' or 'repair' on 2 arrays by
   writing to their md/sync_action attribute files. One operation should
   start and one should be delayed and a message like the above will be
   printed.
3. Issue a write to the third array. Each write will cause 2 copies of
   the message to be printed.

This happens when wake_up(&resync_wait) is called, usually by
md_check_recovery(). Then the delayed sync thread again prints the
message and is put to sleep. This patch adds a check in md_do_sync() to
prevent printing this message more than once for the same pair of
devices.

Reported-by: Sven Koehler <sven.koehler@gmail.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=151801
Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -7865,6 +7865,7 @@ void md_do_sync(struct md_thread *thread)
 	 */
 
 	do {
+		int mddev2_minor = -1;
 		mddev->curr_resync = 2;
 
 	try_again:
@@ -7894,10 +7895,14 @@ void md_do_sync(struct md_thread *thread)
 				prepare_to_wait(&resync_wait, &wq, TASK_INTERRUPTIBLE);
 				if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
 				    mddev2->curr_resync >= mddev->curr_resync) {
-					printk(KERN_INFO "md: delaying %s of %s"
-					       " until %s has finished (they"
-					       " share one or more physical units)\n",
-					       desc, mdname(mddev), mdname(mddev2));
+					if (mddev2_minor != mddev2->md_minor) {
+						mddev2_minor = mddev2->md_minor;
+						printk(KERN_INFO "md: delaying %s of %s"
+						       " until %s has finished (they"
+						       " share one or more physical units)\n",
+						       desc, mdname(mddev),
+						       mdname(mddev2));
+					}
 					mddev_put(mddev2);
 					if (signal_pending(current))
 						flush_signals(current);