Btrfs: empty_size allocation fixes again

The allocator wasn't catching all of the cases where it needed to do
extra loops because the check to enforce them wasn't happening early
enough.

When the allocator decided to increase the size of the allocation
for metadata clustering, it wasn't always setting the empty_size to
include the extra (optional) bytes.  This also fixes the empty_size field
to be correct.

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