btrfs: export global block reserve size as space_info

Introduce a block group type bit for a global reserve and fill the space
info for SPACE_INFO ioctl. This should replace the newly added ioctl
(01e219e8069516cdb98594d417b8bb8d906ed30d) to get just the 'size' part
of the global reserve, while the actual usage can be now visible in the
'btrfs fi df' output during ENOSPC stress.

The unpatched userspace tools will show the blockgroup as 'unknown'.

CC: Jeff Mahoney <jeffm@suse.com>
CC: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.cz>