Btrfs: don't count I/O statistic read errors for missing devices

It is normal behaviour of the low level btrfs function btrfs_map_bio()
to complete a bio with -EIO if the device is missing, instead of just
preventing the bio creation in an earlier step.
This used to cause I/O statistic read error increments and annoying
printk_ratelimited messages. This commit fixes the issue.
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Reported-by: Carey Underwood <cwillu@cwillu.com>

fs/btrfs/volumes.c

@@ -4061,6 +4061,7 @@ static void btrfs_end_bio(struct bio *bio, int err)
 
 			BUG_ON(stripe_index >= bbio->num_stripes);
 			dev = bbio->stripes[stripe_index].dev;
+			if (dev->bdev) {
 				if (bio->bi_rw & WRITE)
 					btrfs_dev_stat_inc(dev,
 						BTRFS_DEV_STAT_WRITE_ERRS);
@@ -4073,6 +4074,7 @@ static void btrfs_end_bio(struct bio *bio, int err)
 				btrfs_dev_stat_print_on_error(dev);
 			}
 		}
+	}
 
 	if (bio == bbio->orig_bio)
 		is_orig_bio = 1;