dma-buf/fence: Fix lock inversion within dma-fence-array

Ages ago Rob Clark noted,

"Currently with fence-array, we have a potential deadlock situation.  If
we fence_add_callback() on an array-fence, the array-fence's lock is
acquired first, and in it's ->enable_signaling() callback, it will install
cbs on it's array-member fences, so the array-member's lock is acquired
second.

But in the signal path, the array-member's lock is acquired first, and
the array-fence's lock acquired second."

Rob proposed either extensive changes to dma-fence to unnest the
fence-array signaling, or to defer the signaling onto a workqueue. This
is a more refined version of the later, that should keep the latency
of the fence signaling to a minimum by using an irq-work, which is
executed asap.

Reported-by: Rob Clark <[email protected]>
Suggested-by: Rob Clark <[email protected]>
References: [email protected]
Signed-off-by: Chris Wilson <[email protected]>
Cc: Rob Clark <[email protected]>
Cc: Gustavo Padovan <[email protected]>
Cc: Sumit Semwal <[email protected]>
Cc: Christian König <[email protected]>
Reviewed-by: Christian König <[email protected]>
Signed-off-by: Sumit Semwal <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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