We need to serialise the addition of a new fence into the shared list
such that the fence is visible before we claim it is there. Otherwise a
concurrent reader of the shared fence list will see an uninitialised
fence slot before it is set.

  <4> [109.613162] general protection fault: 0000 [#1] PREEMPT SMP PTI
  <4> [109.613177] CPU: 1 PID: 1357 Comm: gem_busy Tainted: G     U            4.19.0-rc8-CI-CI_DRM_5035+ #1
  <4> [109.613189] Hardware name: Dell Inc. XPS 8300  /0Y2MRG, BIOS A06 10/17/2011
  <4> [109.613252] RIP: 0010:i915_gem_busy_ioctl+0x146/0x380 [i915]
  <4> [109.613261] Code: 0b 43 04 49 83 c6 08 4d 39 e6 89 43 04 74 6d 4d 8b 3e e8 5d 54 f4 e0 85 c0 74 0d 80 3d 08 71 1d 00 00
  0f 84 bb 00 00 00 31 c0 <49> 81 7f 08 20 3a 2c a0 75 cc 41 8b 97 50 02 00 00 49 8b 8f a8 00
  <4> [109.613283] RSP: 0018:ffffc9000044bcf8 EFLAGS: 00010246
  <4> [109.613292] RAX: 0000000000000000 RBX: ffffc9000044bdc0 RCX: 0000000000000001
  <4> [109.613302] RDX: 0000000000000000 RSI: 00000000ffffffff RDI: ffffffff822474a0
  <4> [109.613311] RBP: ffffc9000044bd28 R08: ffff88021e158680 R09: 0000000000000001
  <4> [109.613321] R10: 0000000000000040 R11: 0000000000000000 R12: ffff88021e1641b8
  <4> [109.613331] R13: 0000000000000003 R14: ffff88021e1641b0 R15: 6b6b6b6b6b6b6b6b
  <4> [109.613341] FS:  00007f9c9fc84980(0000) GS:ffff880227a40000(0000) knlGS:0000000000000000
  <4> [109.613352] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  <4> [109.613360] CR2: 00007f9c9fcb8000 CR3: 00000002247d4005 CR4: 00000000000606e0

Fixes: 27836b64

 ("dma-buf: remove shared fence staging in reservation object")
Testcase: igt/gem_busy/close-race
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Cc: Junwei Zhang <Jerry.Zhang@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181026080302.11507-1-chris@chris-wilson.co.uk