Fixes the BUG_ON spuriously triggering under the following
circumstances:

* reservation_object_reserve_shared is called with shared_count ==
  shared_max - 1, so obj->staged is freed in preparation of an in-place
  update.

* reservation_object_add_shared_fence is called with the first fence,
  after which shared_count == shared_max.

* reservation_object_add_shared_fence is called with a follow-up fence
  from the same context.

In the second reservation_object_add_shared_fence call, the BUG_ON
triggers. However, nothing bad would happen in
reservation_object_add_shared_inplace, since both fences are from the
same context, so they only occupy a single slot.

Prevent this by moving the BUG_ON to where an overflow would actually
happen (e.g. if a buggy caller didn't call
reservation_object_reserve_shared before).

v2:
* Fix description of breaking scenario (Christian König)
* Add bugzilla reference

Cc: stable@vger.kernel.org
Bugzilla: https://bugs.freedesktop.org/106418
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> # v1
Reviewed-by: Christian König <christian.koenig@amd.com> # v1
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180704151405.10357-1-michel@daenzer.net