btrfs: add and use helper to compute the available space for a block group

btrfs: add and use helper to compute the available space for a block group

We have currently three places that compute how much available space a
block group has. Add a helper function for this and use it in those
places.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>