rbtree: Add support for augmented rbtrees

Add support for augmented rbtrees in core rbtree code.

This will be used in subsequent patches, in x86 PAT code, which needs
interval trees to efficiently keep track of PAT ranges.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
LKML-Reference: <20100210232343.GA11465@linux-os.sc.intel.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>