ondemand readahead simplifications

Andrew,

These two patches makes the interface and data structure of ondemand readahead more clear:

diffstat:

 fs/ext3/dir.c      |    4 -
 fs/ext4/dir.c      |    4 -
 fs/splice.c        |    6 -
 include/linux/fs.h |   61 +--------------
 include/linux/mm.h |   20 +++--
 mm/filemap.c       |   10 +-
 mm/readahead.c     |  165 +++++++++++++++++++++----------------------
 7 files changed, 116 insertions(+), 154 deletions(-)

Regards,
Fengguang