dm bufio: add LRU abstraction

A CLOCK algorithm is used in this LRU abstraction.  This avoids
relinking list nodes, which would require a write lock protecting it.

None of the LRU methods are threadsafe; locking must be done at a
higher level.

Code that uses this new LRU will be introduced in the next 2 commits.

As such, this commit will cause "defined but not used" compiler warnings
that will be resolved by the next 2 commits.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

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