btrfs-progs: mkfs should check for small vol well before

btrfs-progs: mkfs should check for small vol well before

This fix the regression introduced by 830427d
that it no more creates the FS if disk is small
and if no mixed option is provided.
This patch will bring it to the original design
which will force mixed profile when disk is small
and go ahead to create the FS.

Which also means that before we open the device
for the write we should also check if disk is small.

v2: fixes the checkpatch.pl warnings

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