dma-buf: protected fence ops by RCU v8

dma-buf: protected fence ops by RCU v8

The fence ops of a dma_fence currently need to life as long as the
dma_fence is alive. This means that the module which originally issued
a dma_fence can't unload unless all fences are freed up.

As first step to solve this issue protect the fence ops by RCU.

While it is counter intuitive to protect a constant function pointer table
by RCU it allows modules to wait for an RCU grace period before they
unload, to make sure that nobody is executing their functions any more.

This patch has not much functional change, but only adds the RCU
handling for the static checker to test.

v2: make one the now duplicated lockdep warnings a comment instead.
v3: Add more documentation to ->wait and ->release callback.
v4: fix typo in documentation
v5: rebased on drm-tip
v6: improve code comments
v7: improve commit message and code comments
v8: fix sparse rcu warnings

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/r/20260219160822.1529-2-christian.koenig@amd.com