idr_get_new() currently returns an incrementing counter in the top 8 bits of
the counter.  Which means that most users have to mask it off again, and we
only have a 24-bit range.

So remove that counter.  Also:

- Remove the BITS_PER_INT define due to namespace collision risk.

- Make MAX_ID_SHIFT 31, so counters have a 0 to 2G-1 range.

- Why is MAX_ID_SHIFT using sizeof(int) and not sizeof(long)?  If it's for
  consistency across 32- and 64-bit machines, why not just make it "31"?

- Does this still hold true with the counter removed?

/* We can only use half the bits in the top level because there are
   only four possible bits in the top level (5 bits * 4 levels = 25
   bits, but you only use 24 bits in the id). */

  If not, what needs to change?
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>