fix potential race condition in canny.cl.

fix potential race condition in canny.cl.

See the below code snippet:

while(l_counter != 0)
{
    int mod = l_counter % LOCAL_TOTAL;
    int pix_per_thr = l_counter / LOCAL_TOTAL + ((lid < mod) ? 1 : 0);

    for (int i = 0; i < pix_per_thr; ++i)
    {
        int index = atomic_dec(&l_counter) - 1;
        ....
    }
    ....
    barrier(CLK_LOCAL_MEM_FENCE);
}

If we don't put a barrier before the for loop, then there is a possiblity
that some work item enter this loop but the others are not, the the l_counter
will be reduced in the for loop and may be changed to zero, and the other
work items may can't enter the while loop. If this happens, it breaks the
barrier's rule which requires all the work items reach the same barrier.
And it may hang the GPU depends on the implementation of opencl platform.

This issue is raised at:
https://github.com/Itseez/opencv/issues/5175

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>