btrfs: convert BUG_ON() in btrfs_truncate_inode_items to ASSERT

We have a correctness BUG_ON() in btrfs_truncate_inode_items to make
sure that we're always using min_type == BTRFS_EXTENT_DATA_KEY if
new_size is > 0.  Convert this to an ASSERT.
Reviewed-by: Filipe Manana <fdmanana@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/inode-item.c

@@ -476,7 +476,7 @@ int btrfs_truncate_inode_items(struct btrfs_trans_handle *trans,
 	bool should_throttle = false;
 
 	ASSERT(control->inode || !control->clear_extent_range);
-	BUG_ON(new_size > 0 && control->min_type != BTRFS_EXTENT_DATA_KEY);
+	ASSERT(new_size == 0 || control->min_type == BTRFS_EXTENT_DATA_KEY);
 
 	control->last_size = new_size;
 	control->sub_bytes = 0;