btrfs-progs: fix wrong message about forcing the mixed group profile

btrfs-progs: fix wrong message about forcing the mixed group profile

When the added disk is small, the function btrfs_prepare_device() would
print message indicating that mixed group is forced, apparently thats
not true

reproducer:

mkfs.btrfs -f -draid1 -mraid1 /dev/sdd /dev/sdc
::
SMALL VOLUME: forcing mixed metadata/data groups <---- WRONG

mount /dev/sdd /btrfs

btrfs fi df /btrfs
Data, RAID1: total=126.25MiB, used=128.00KiB
Data, single: total=8.00MiB, used=0.00B
System, RAID1: total=8.00MiB, used=16.00KiB
System, single: total=4.00MiB, used=0.00B
Metadata, RAID1: total=126.25MiB, used=112.00KiB
Metadata, single: total=8.00MiB, used=0.00B
GlobalReserve, single: total=16.00MiB, used=0.00B

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.cz>