xfstests: make _scratch_mkfs_blocksized usable

xfstests: make _scratch_mkfs_blocksized usable

The default mkfs.xfs options contain -b size=4096, so all tests
using _scratch_mkfs_blocksized won't actually run unless those
options are changed.  As we're trying to specificly test 1k
blocks we should always override the default option.

v2: Move the function to common/rc

Signed-off-by: Christoph Hellwig <hch@lst.de>
[darrick.wong@oracle.com: move function to common/rc]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>