FROMGIT: dma-buf: insert memory barrier before updating num_fences

smp_store_mb() inserts memory barrier after storing operation.
It is different with what the comment is originally aiming so Null
pointer dereference can be happened if memory update is reordered.

Signed-off-by: Hyejeong Choi <hjeong.choi@samsung.com>

Bug: 417615398
(cherry picked from commit 72c7d625
 https://gitlab.freedesktop.org/drm/misc/kernel.git

 drm-misc-fixes)
Change-Id: I7a984743f929fb8120d0efcf991087e4f8d489ae
Signed-off-by: Hyejeong Choi <hjeong.choi@samsung.com>