readahead: remove the limit max_sectors_kb imposed on max_readahead_kb

Remove the size limit max_sectors_kb imposed on max_readahead_kb.

The size restriction is unreasonable.  Especially when max_sectors_kb cannot
grow larger than max_hw_sectors_kb, which can be rather small for some disk
drives.

Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Acked-by: Jens Axboe <jens.axboe@oracle.com>
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>