btrfs: convert some BUG_ON()'s to ASSERT()'s in do_relocation

A few of these are checking for correctness, and won't be triggered by
corrupted file systems, so convert them to ASSERT() instead of BUG_ON()
and add a comment explaining their existence.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -2181,7 +2181,11 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 	int slot;
 	int ret = 0;
 
-	BUG_ON(lowest && node->eb);
+	/*
+	 * If we are lowest then this is the first time we're processing this
+	 * block, and thus shouldn't have an eb associated with it yet.
+	 */
+	ASSERT(!lowest || !node->eb);
 
 	path->lowest_level = node->level + 1;
 	rc->backref_cache.path[node->level] = node;
@@ -2266,7 +2270,11 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 			free_extent_buffer(eb);
 			if (ret < 0)
 				goto next;
-			BUG_ON(node->eb != eb);
+			/*
+			 * We've just COWed this block, it should have updated
+			 * the correct backref node entry.
+			 */
+			ASSERT(node->eb == eb);
 		} else {
 			btrfs_set_node_blockptr(upper->eb, slot,
 						node->eb->start);
@@ -2302,7 +2310,12 @@ static int do_relocation(struct btrfs_trans_handle *trans,
 	}
 
 	path->lowest_level = 0;
-	BUG_ON(ret == -ENOSPC);
+
+	/*
+	 * We should have allocated all of our space in the block rsv and thus
+	 * shouldn't ENOSPC.
+	 */
+	ASSERT(ret != -ENOSPC);
 	return ret;
 }