bcache: set largest seq to ja->seq[bucket_index] in journal_read_bucket()

In journal_read_bucket() when setting ja->seq[bucket_index], there might
be potential case that a later non-maximum overwrites a better sequence
number to ja->seq[bucket_index]. This patch adds a check to make sure
that ja->seq[bucket_index] will be only set a new value if it is bigger
then current value.
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/journal.c

@@ -156,6 +156,7 @@ reread:		left = ca->sb.bucket_size - offset;
 			list_add(&i->list, where);
 			ret = 1;
 
+			if (j->seq > ja->seq[bucket_index])
 				ja->seq[bucket_index] = j->seq;
 next_set:
 			offset	+= blocks * ca->sb.block_size;