IB/hfi1: Add evict operation to the mmu rb handler

Allow users to clear nodes from the rb tree based on their evict callback.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dean Luick <dean.luick@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>