ocfs2: Little refactoring against ocfs2_iget.
ocfs2_iget is used to get/create inode. Only iget5_locked will give us an inode = NULL. So move this check ahead of ocfs2_read_locked_inode so that we don't need to check inode before we read and unlock inode. This is also helpful for trace event(see the next patch). Signed-off-by: Tao Ma <email@example.com>