btrfs: don't check for btrfs_device::bdev in btrfs_end_bio

btrfs_map_bio ensures that all submitted bios to devices have valid
btrfs_device::bdev so this check can be removed from btrfs_end_bio. This
check was added in june 2012 597a60fa ("Btrfs: don't count I/O
statistic read errors for missing devices") but then in October of the
same year another commit de1ee92a ("Btrfs: recheck bio against
block device when we map the bio") started checking for the presence of
btrfs_device::bdev before actually issuing the bio.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6263,7 +6263,7 @@ static void btrfs_end_bio(struct bio *bio)
 		    bio->bi_status == BLK_STS_TARGET) {
 			struct btrfs_device *dev = btrfs_io_bio(bio)->device;
 
-			if (dev->bdev) {
+			ASSERT(dev->bdev);
 			if (bio_op(bio) == REQ_OP_WRITE)
 				btrfs_dev_stat_inc_and_print(dev,
 						BTRFS_DEV_STAT_WRITE_ERRS);
@@ -6275,7 +6275,6 @@ static void btrfs_end_bio(struct bio *bio)
 						BTRFS_DEV_STAT_FLUSH_ERRS);
 		}
 	}
-	}
 
 	if (bio == bbio->orig_bio)
 		is_orig_bio = 1;