The chunk size is stored in the btrfs_space_info structure.  It is
initialized at the start and is then used.

A new API is added to update the current chunk size.  This API is used
to be able to expose the chunk_size as a sysfs setting.
Signed-off-by: NStefan Roesch <shr@fb.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
[ rename and merge helpers, switch atomic type to u64, style fixes ]
Signed-off-by: NDavid Sterba <dsterba@suse.com>