ocfs2/refcounttree: add the missing NULL check of the return value of find_or_create_...

Add the missing NULL check of the return value of find_or_create_page() in
function ocfs2_duplicate_clusters_by_page().

[akpm@linux-foundation.org: fix layout, per Joel]
Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Acked-by: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>