This adds a cache of lastest accessed cluster to inode for sequential
access.

The following is 500M file of FAT-to-FAT copy test, this may be a most
different case in usual operations, because maximum readahead window
flush the all caches.

512 bytes blocksize, 4096 bytes cluster size.

linux-2.5.69-bk12
	root@devron (a)[1232]# time cp file file1

	real    7m58.900s
	user    0m0.267s
	sys     6m44.258s

linux-2.5.69-bk12+patch
	root@devron (a)[1576]# time cp file file1

	real    2m44.309s
	user    0m0.270s
	sys     0m28.631s