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 <boyu.mt@taobao.com>