Btrfs: revamp clustered allocation logic

Parameterize clusters on minimum total size, minimum chunk size and
minimum contiguous size for at least one chunk, without limits on
cluster, window or gap sizes.  Don't tolerate any fragmentation for
SSD_SPREAD; accept it for metadata, but try to keep data dense.

Signed-off-by: Alexandre Oliva <oliva@lsd.ic.unicamp.br>
Signed-off-by: Chris Mason <chris.mason@oracle.com>