idr: reorder the fields

idr: reorder the fields

idr_layer->layer is always accessed in read path, move it in the front.

idr_layer->bitmap is moved on the bottom.  And rcu_head shares with
bitmap due to they do not be accessed at the same time.

idr->id_free/id_free_cnt/lock are free list fields, and moved to the
bottom.  They will be removed in near future.

Signed-off-by: Lai Jiangshan <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>