Btrfs-progs: add support for mixed data+metadata block groups

Btrfs-progs: add support for mixed data+metadata block groups

So alot of crazy people (I'm looking at you Meego) want to use btrfs on phones
and such with small devices.  Unfortunately the way we split out metadata/data
chunks it makes space usage inefficient for volumes that are smaller than
1gigabyte.  So add a -M option for mixing metadata+data, and default to this
mixed mode if the filesystem is less than or equal to 1 gigabyte.  I've tested
this with xfstests on a 100mb filesystem and everything is a-ok.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>