fat: improve the readahead for FAT entries

fat: improve the readahead for FAT entries

Current readahead for FAT entries is very simple but is having some flaws,
so it is not working well for some environments.  This patch improves the
readahead more or less.

The key points of modification are,

  - make the readahead size tunable by using bdi->ra_pages
  - care the bdi->io_pages to avoid the small size I/O request
  - update readahead window before fully exhausting

With this patch, on slow USB connected 2TB hdd:

[before]
383.18sec

[after]
51.03sec

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: hyeongseok.kim <hyeongseok.kim@lge.com>
Reviewed-by: hyeongseok.kim <hyeongseok.kim@lge.com>
Link: http://lkml.kernel.org/r/87d08e1dlh.fsf@mail.parknet.co.jp
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>