btrfs: Fix threshold calculation for block groups smaller than 1GB

If a block group is smaller than 1GB, the extent entry threadhold
calculation will always set the threshold to 0.

So as free space gets fragmented, btrfs will switch to use bitmap
to manage free space, but then will never switch back to extents
due to this bug.

Reviewed-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>