generic/015: fix to test the default non-mixed mode

generic/015: fix to test the default non-mixed mode

commit 97575acd7495 (generic/015: Change the test filesystem size to
101mb), created 101mb FS instead of 100mb FS to make sure we create
a FS which is non mixed mode.

btrfs-progs commit 18e2663db3e1 (btrfs-progs: Add minimum device
size check) added a more accurate minimum required space to create
the btrfs FS in non mixed mode depending on the group profile, and
considering any group profiles we would need at least 256MB (with
upward round off).

So this patch changes the FS size to be created by
_scratch_sized_mkfs() to 256MB so that we create the FS in non mixed
mode for any group profile.

Mixed blockgroup can be tested using the MKFS_OPTIONS explicitly.

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