btrfs: add minimal file system size check

btrfs: add minimal file system size check

Some btrfs tests call _scratch_pool_mkfs or _scratch_mkfs by themselves to
specify file system size limit. It slips through the check in
_scratch_mkfs_sized(). Let's add size check call for each of them.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>