idr: fix NULL pointer dereference when ida_remove(unallocated_id)

If the ida has at least one existing id, and when an unallocated ID
which meets a certain condition is passed to the ida_remove(), the
system will crash because it hits NULL pointer dereference.

The condition is that the unallocated ID shares the same lowest idr
layer with the existing ID, but the idr slot would be different if the
unallocated ID were to be allocated.

In this case the matching idr slot for the unallocated_id is NULL,
causing @bitmap to be NULL which the function dereferences without
checking crashing the kernel.

See the test code:

  static void test3(void)
  {
        int id;
        DEFINE_IDA(test_ida);

        printk(KERN_INFO "Start test3\n");
        if (ida_pre_get(&test_ida, GFP_KERNEL) < 0) return;
        if (ida_get_new(&test_ida,  &id) < 0) return;
        ida_remove(&test_ida, 4000); /* bug: null deference here */
        printk(KERN_INFO "End of test3\n");
  }

It happens only when the caller tries to free an unallocated ID which is
the caller's fault.  It is not a bug.  But it is better to add the
proper check and complain rather than crashing the kernel.

[tj@kernel.org: updated patch description]
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>