linear currently uses division by the size of the smallest componenet device
to find which device a request goes to.  If that smallest device is larger
than 2 terabytes, then the division will not work on some systems.

So we introduce a pre-shift, and take care not to make the hash table too
large, much like the code in raid0.

Also get rid of conf->nr_zones, which is not needed.
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>