Btrfs: don't force chunk allocation in find_free_extent

find_free_extent likes to allocate in contiguous clusters,
which makes writeback faster, especially on SSD storage.  As
the FS fragments, these clusters become harder to find and we have
to decide between allocating a new chunk to make more clusters
or giving up on the cluster to allocate from the free space
we have.

Right now it creates too many chunks, and you can end up with
a whole FS that is mostly empty metadata chunks.  This commit
changes the allocation code to be more strict and only
allocate new chunks when we've made good use of the chunks we
already have.

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