btrfs: support other sectorsizes in _scratch_mkfs_blocksized

When btrfs supports sectorsize != pagesize it can run these test cases
now,
generic/205 generic/206 generic/216 generic/217 generic/218 generic/220
generic/222 generic/227 generic/229 generic/238

This change is backward compatible for kernels without non pagesize
sectorsize support.

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