Add a field which carries the prefix of ID the idr_layer covers. This
will be used to implement lookup hint.
This patch doesn't make use of the new field and doesn't introduce any
behavior difference.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>