The hashtable that linear uses to find the right device stores
two pointers for every entry.

The second is always one of:
   The first plus 1
   NULL
When NULL, it is never accessed, so any value can be stored.

Thus it could always be "first plus 1", and so we don't need to store
it as it is trivial to calculate.

This patch halves the size of this table, which results in some simpler
code as well.
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>