zcache: convert to idr_alloc()

idr_get_new*() and friends are about to be deprecated.  Convert to the
new idr_alloc() interface.

Only compile tested.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>