dma-buf: test shared slot allocation when mutex debugging is active

Set shared_max to the number of shared fences right before we release
the lock.

This way every attempt to add a shared fence without previously
reserving a slot will cause an error.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Junwei Zhang <Jerry.Zhang@amd.com>
Link: https://patchwork.kernel.org/patch/10626147/