Btrfs: Fix more false enospc errors and an oops from empty clustering

Btrfs: Fix more false enospc errors and an oops from empty clustering

In comes cases the empty cluster was added twice to the total number of
bytes the allocator was trying to find.

With empty clustering on, the hint byte was sometimes outside of the
block group.  Add an extra goto to find the correct block group.

Signed-off-by: Chris Mason <chris.mason@oracle.com>