ocfs2/reflink: fix file block size reporting

ocfs2/reflink: fix file block size reporting

Some of the reflink tests try to require a specific filesystem block
size so that they can test file block manipulation functions.  That's
straightforward for most filesystems but ocfs2 throws in the additional
twist that data fork block mappings are stored in units of clusters, not
blocks, which causes these reflink tests to fail.

Therefore, introduce a new helper that retrieves the file minimum block
size and adapt the reflink tests to use that instead.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>