ocfs2: optimize ocfs2_check_dir_entry() with unlikely() annotations

In cad3f00, ext4_check_dir_entry was modified by adding some unlikely.
Ted described it as "This function gets called a lot for large
directories, and the answer is almost always 'no, no, there's no problem'.
This means using unlikely() is a good thing."
ext3 added the similar change in commit a4ae309.

So change it accordingly in ocfs2.

Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Joel Becker <jlbec@evilplan.org>