btrfs: move btrfs_chunk_item_size out of ctree.h

This is used by the volumes code and the tree checker code.  We want to
maintain inline however, so simply move it to volumes.h.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>