readahead: compacting file_ra_state

Use 'unsigned int' instead of 'unsigned long' for readahead sizes.

This helps reduce memory consumption on 64bit CPU when a lot of files are
opened.

CC: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>