IB/hfi1: Add SDMA cache eviction algorithm

This commit adds a cache eviction algorithm for the SDMA
user buffer cache.

Besides the interval RB tree used for node lookup, the cache
nodes are also arranged in a doubly-linked list. When a node is
used, it is put at the beginning of the list. Less frequently
used nodes naturally move to the tail of the list.

When the cache limit is reached, the eviction code starts
traversing the linked list in reverse, freeing buffers until
enough space has been freed to fit the new user buffer. This
guarantees that only the least used cache nodes will be removed
from the cache.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Reviewed-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Jubin John <jubin.john@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>