md/raid1: fix read balance when a drive is write-mostly.

When a drive is marked write-mostly it should only be the
target of reads if there is no other option.

This behaviour was broken by

commit 9dedf603
    md/raid1: read balance chooses idlest disk for SSD

which causes a write-mostly device to be *preferred* is some cases.

Restore correct behaviour by checking and setting
best_dist_disk and best_pending_disk rather than best_disk.

We only need to test one of these as they are both changed
from -1 or >=0 at the same time.

As we leave min_pending and best_dist unchanged, any non-write-mostly
device will appear better than the write-mostly device.
Reported-by: NTomáš Hodek <tomas.hodek@volny.cz>
Reported-by: NDark Penguin <darkpenguin@yandex.ru>
Signed-off-by: NNeilBrown <neilb@suse.de>
Link: http://marc.info/?l=linux-raid&m=135982797322422
Fixes: 9dedf603
Cc: stable@vger.kernel.org (3.6+)

drivers/md/raid1.c

@@ -560,7 +560,7 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 		if (test_bit(WriteMostly, &rdev->flags)) {
 			/* Don't balance among write-mostly, just
 			 * use the first as a last resort */
-			if (best_disk < 0) {
+			if (best_dist_disk < 0) {
 				if (is_badblock(rdev, this_sector, sectors,
 						&first_bad, &bad_sectors)) {
 					if (first_bad < this_sector)
@@ -569,7 +569,8 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 					best_good_sectors = first_bad - this_sector;
 				} else
 					best_good_sectors = sectors;
-				best_disk = disk;
+				best_dist_disk = disk;
+				best_pending_disk = disk;
 			}
 			continue;
 		}