ocfs2: clean up unused variable in ocfs2_duplicate_clusters_by_page()

readahead_pages in ocfs2_duplicate_clusters_by_page is defined but not
used, so clean it up.

Signed-off-by: Joseph Qi <joseph.qi@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>