btrfs: _scratch_mkfs_sized fix min size without mixed option

btrfs: _scratch_mkfs_sized fix min size without mixed option

As of now _scratch_mkfs_sized() checks if the requested size is
below 1G and forces the --mixed option for the mkfs.btrfs. Well the
correct size considering all possible group profiles at which we
need to force the mixed option is roughly 256Mbytes. So fix that.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>