ocfs2: implement directory read-ahead

Uptodate.c now knows about read-ahead buffers. Use some more aggressive
logic in ocfs2_readdir().

The two functions which currently use directory read-ahead are
ocfs2_find_entry() and ocfs2_readdir().

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

diff --cc fs/ocfs2/buffer_head_io.c
Simple merge

diff --cc fs/ocfs2/buffer_head_io.h
Simple merge

diff --cc fs/ocfs2/dir.c
Simple merge

diff --cc fs/ocfs2/inode.c
Simple merge

diff --cc fs/ocfs2/namei.c
Simple merge

diff --cc fs/ocfs2/uptodate.c
Simple merge

diff --cc fs/ocfs2/uptodate.h
Simple merge