xfs: refactor minimum log size formatting code

xfs: refactor minimum log size formatting code

Create a new helper function to discover the minimum log size that will
work with the mkfs options provided, then remove all the hardcoded block
sizes from various xfs tests.  This will be necessary when we turn on
reflink or rmap by default and the minimum log size increases.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Tested-by: Yang Xu<xuyang2018.jy@cn.fujitsu.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>