drm/ttm: Provide a shmem backup implementation

Provide a standalone shmem backup implementation.
Given the ttm_backup interface, this could
later on be extended to providing other backup
implementation than shmem, with one use-case being
GPU swapout to a user-provided fd.

v5:
- Fix a UAF. (kernel test robot, Dan Carptenter)
v6:
- Rename ttm_backup_shmem_copy_page() function argument
  (Matthew Brost)
- Add some missing documentation
v8:
- Use folio_file_page to get to the page we want to writeback
  instead of using the first page of the folio.
v13:
- Remove the base class abstraction (Christian König)
- Include ttm_backup_bytes_avail().
v14:
- Fix kerneldoc for ttm_backup_bytes_avail() (0-day)
- Work around casting of __randomize_layout struct pointer (0-day)
v15:
- Return negative error code from ttm_backup_backup_page()
  (Christian König)
- Doc fixes. (Christian König).

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

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