idr: fix overflow bug during maximum ID calculation at maximum height

idr: fix overflow bug during maximum ID calculation at maximum height

idr_replace() open-codes the logic to calculate the maximum valid ID given
the height of the idr tree; unfortunately, the open-coded logic doesn't
account for the fact that the top layer may have unused slots and
over-shifts the limit to zero when the tree is at its maximum height.

The following test code shows it fails to replace the value for
id=((1<<27)+42):

static void test5(void)
{
	int id;
	DEFINE_IDR(test_idr);
#define TEST5_START ((1<<27)+42) /* use the highest layer */

	printk(KERN_INFO "Start test5\n");
	id = idr_alloc(&test_idr, (void *)1, TEST5_START, 0, GFP_KERNEL);
	BUG_ON(id != TEST5_START);
	TEST_BUG_ON(idr_replace(&test_idr, (void *)2, TEST5_START) != (void *)1);
	idr_destroy(&test_idr);
	printk(KERN_INFO "End of test5\n");
}

Fix the bug by using idr_max() which correctly takes into account the
maximum allowed shift.

sub_alloc() shares the same problem and may incorrectly fail with -EAGAIN;
however, this bug doesn't affect correct operation because
idr_get_empty_slot(), which already uses idr_max(), retries with the
increased @id in such cases.

[tj@kernel.org: Updated patch description.]
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>