btrfs: publish allocation data in sysfs

While trying to debug ENOSPC issues, it's helpful to understand what the
kernel's view of the available space is. We export this information
via ioctl, but sysfs files are more easily used.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>