md: never clear bit from the write-intent bitmap when the array is degraded.

It is safe to clear a bit from the write-intent bitmap for a raid1
if we know the data has been written to all devices, which is
what the current test does.

But it is not always safe to update the 'events_cleared' counter in
that case.  This is because one request could complete successfully
after some other request has partially failed.

So simply disable the clearing and updating of events_cleared whenever
the array is degraded.  This might end up not clearing some bits that
could safely be cleared, but it is safest approach.

Note that the bug fixed here did not risk corrupting data by letting
the array get out-of-sync.  Rather it meant that when a device is
removed and re-added to the array, it might incorrectly require a full
recovery rather than just recovering based on the bitmap.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/bitmap.c

@@ -1307,6 +1307,9 @@ void bitmap_endwrite(struct bitmap *bitmap, sector_t offset, unsigned long secto
 		PRINTK(KERN_DEBUG "dec write-behind count %d/%d\n",
 		  atomic_read(&bitmap->behind_writes), bitmap->max_write_behind);
 	}
+	if (bitmap->mddev->degraded)
+		/* Never clear bits or update events_cleared when degraded */
+		success = 0;
 
 	while (sectors) {
 		int blocks;