xfs: add multi link parent pointer test

xfs: add multi link parent pointer test

Add a test to verify parent pointers while multiple links to a file are
created and removed.

Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Catherine Hoang <catherine.hoang@oracle.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>