ocfs2: Access the right buffer_head in ocfs2_merge_rec_left.

In commit "ocfs2: Use metadata-specific ocfs2_journal_access_*()
functions", the wrong buffer_head is accessed. So change it
to the right buffer_head.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Acked-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>