Btrfs: fix data enospc check overflow

Because we account for reserved space we get from the allocator before we
actually account for allocating delalloc space, we can have a small window where
the amount of "used" space in a space_info is more than the total amount of
space in the space_info.  This will cause a overflow in our check, so it will
seem like we have _tons_ of free space, and we'll allow reservations to occur
that will end up larger than the amount of space we have.  I've seen users
report ENOSPC panic's in cow_file_range a few times recently, so I tried to
reproduce this problem and found I could reproduce it if I ran one of my tests
in a loop for like 20 minutes.  With this patch my test ran all night without
issues.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>