drm/i915: Do not sync RCU during shrinking

Due to the complex dependencies between workqueues and RCU, which
are not easily detected by lockdep, do not synchronize RCU during
shrinking.

On low-on-memory systems (mem=1G for example), the RCU sync leads
to all system workqueus freezing and unrelated lockdep splats are
displayed according to reports. GIT bisecting done by J. R.
Okajima points to the commit where RCU syncing was extended.

RCU sync gains us very little benefit in real life scenarios
where the amount of memory used by object backing storage is
dominant over the metadata under RCU, so drop it altogether.

 " Yeeeaah, if core could just, go ahead and reclaim RCU
   queues, that'd be great. "

  - Chris Wilson, 2016 (0eafec6d3244)

v2: More information to commit message.
v3: Remove "grep _rcu_" escapee from i915_gem_shrink_all (Andrea)

Fixes: c053b5a506d3 ("drm/i915: Don't call synchronize_rcu_expedited under struct_mutex")
Suggested-by: Chris Wilson <[email protected]>
Reported-by: J. R. Okajima <[email protected]>
Signed-off-by: Joonas Lahtinen <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Tested-by: Hugh Dickins <[email protected]>
Tested-by: Andrea Arcangeli <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: J. R. Okajima <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: <[email protected]> # v4.11+
(cherry picked from commit 73cc0b9aa9afa5ba65d92e46ded61d29430d72a4)
Signed-off-by: Jani Nikula <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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