Btrfs: lower the bar for chunk allocation

The chunk allocation code has tried to keep a pretty tight lid on creating new
metadata chunks.  This is partially because in the past the reservation
code didn't give us an accurate idea of how much space was being used.

The new code is much more accurate, so we're able to get rid of some of these
checks.

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