btrfs-progs: mkfs: create only desired block groups for single device

btrfs-progs: mkfs: create only desired block groups for single device

The filesystem creation has to solve some chicken-egg problems and
creates some temporary objects. In our case it's an extra single/single
pair of block groups that's not used unless the user asks that
explicitly.

Example:

Data, single: total=8.00MiB, used=64.00KiB
System, DUP: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, DUP: total=153.56MiB, used=112.00KiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B

Even with a single device filesystem and defaults, there's single
block group for metadata and system. The single device case is easy to
fix, we'll simply create the right type from the beginning.

Example:

Data, single: total=8.00MiB, used=64.00KiB
System, DUP: total=4.00MiB, used=16.00KiB
Metadata, DUP: total=136.00MiB, used=112.00KiB
GlobalReserve, single: total=16.00MiB, used=0.00B

Filesystem on top of multiple devices still leaves the single/single
groups behind.

Signed-off-by: David Sterba <dsterba@suse.com>