Add a simple LRU helper to assist with driver's shrinker implementation.
It handles tracking the number of backing pages associated with a given
LRU, and provides a helper to implement shrinker_scan.

A driver can use multiple LRU instances to track objects in various
states, for example a dontneed LRU for purgeable objects, a willneed LRU
for evictable objects, and an unpinned LRU for objects without backing
pages.

All LRUs that the object can be moved between must share a single lock.

v2: lockdep_assert_held() instead of WARN_ON(!mutex_is_locked())
v3: make drm_gem_lru_move_tail_locked() static until there is a user

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Patchwork: https://patchwork.freedesktop.org/patch/496128/
Link: https://lore.kernel.org/r/20220802155152.1727594-10-robdclark@gmail.com