IB/hfi1: Use interval RB trees

The interval RB trees can handle RB nodes which
hold ranged information. This is exactly the usage
for the buffer cache implemented in the expected
receive code path.

Convert the MMU/RB functions to use the interval RB
tree API. This will help with future users of the
caching API, as well.

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>