reflink: don't test disjoint block sharing sets

reflink: don't test disjoint block sharing sets

Unlike xfs/btrfs which store refcounting information as part of the
space metadata, ocfs2 implements block sharing (reflink) by creating
refcount btrees that are shared between subsets of files.  Effectively,
this means that a ocfs2 can have multiple disjoint sets of files that
share blocks, which also means that blocks cannot be reflinked between
two disjoint refcounted-file-sets.  generic/119 tests the ability to do
this, so we cannot run it for ocfs2.  Create a _require helper to check
for this.

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>