drm/ttm: Add a macro to perform LRU iteration

Following the design direction communicated here:

https://lore.kernel.org/linux-mm/[email protected]/T/#ma918844aa8a6efe8768fdcda0c6590d5c93850c9

Export a LRU walker for driver shrinker use. The walker
initially supports only trylocking, since that's the
method used by shrinkes. The walker makes use of
scoped_guard() to allow exiting from the LRU walk loop
without performing any explicit unlocking or
cleanup.

v8:
- Split out from another patch.
- Use a struct for bool arguments to increase readability (Matt Brost).
- Unmap user-space cpu-mappings before shrinking pages.
- Explain non-fatal error codes (Matt Brost)

v10:
- Instead of using the existing helper, Wrap the interface inside out and
  provide a loop to de-midlayer things the LRU iteration (Christian König).
- Removing the R-B by Matt Brost since the patch was significantly changed.

v11:
- Split the patch up to include just the LRU walk helper.

v12:
- Indent after scoped_guard() (Matt Brost)

v15:
- Adapt to new definition of scoped_guard()

Signed-off-by: Thomas Hellström <[email protected]>
Reviewed-by: Matthew Brost <[email protected]>
Acked-by: Dave Airlie <[email protected]>
Acked-by: Christian König <[email protected]>
Link: https://lore.kernel.org/intel-xe/[email protected]

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