btrfs: replace BUG_ON with ASSERT in btrfs_read_node_slot

In btrfs_read_node_slot() we have a BUG_ON() that can be converted to an
ASSERT(), it's from an extent buffer and the level is validated at the
time it's read from disk.
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.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/ctree.c

@@ -959,7 +959,7 @@ struct extent_buffer *btrfs_read_node_slot(struct extent_buffer *parent,
 	if (slot < 0 || slot >= btrfs_header_nritems(parent))
 		return ERR_PTR(-ENOENT);
 
-	BUG_ON(level == 0);
+	ASSERT(level);
 
 	check.level = level - 1;
 	check.transid = btrfs_node_ptr_generation(parent, slot);