GFS2: Introduce helper for clearing gl_object

This patch introduces a new helper function in glock.h that
clears gl_object, with an added integrity check. An additional
integrity check has been added to glock_set_object, plus comments.
This is step 1 in a series to ensure gl_object integrity.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
diff --git a/fs/gfs2/inode.c b/fs/gfs2/inode.c
index f9302f1..2578bd8 100644
--- a/fs/gfs2/inode.c
+++ b/fs/gfs2/inode.c
@@ -201,14 +201,14 @@ struct inode *gfs2_inode_lookup(struct super_block *sb, unsigned int type,
 
 fail_refresh:
 	ip->i_iopen_gh.gh_flags |= GL_NOCACHE;
-	glock_set_object(ip->i_iopen_gh.gh_gl, NULL);
+	glock_clear_object(ip->i_iopen_gh.gh_gl, ip);
 	gfs2_glock_dq_uninit(&ip->i_iopen_gh);
 fail_put:
 	if (io_gl)
 		gfs2_glock_put(io_gl);
 	if (gfs2_holder_initialized(&i_gh))
 		gfs2_glock_dq_uninit(&i_gh);
-	glock_set_object(ip->i_gl, NULL);
+	glock_clear_object(ip->i_gl, ip);
 fail:
 	iget_failed(inode);
 	return ERR_PTR(error);