Third generation deduplication tree implementation

This time based on the kernel red-black tree, where nodes
store pointers to its parent node hence allowing for
removals without key comparison if node to be removed is
known.

Also in this patch, a modification to struct page is
included. With it, deduplicated pages don't have mapping
pointer but a pointer to its deduplication tree node.

Signed-off-by: Jacobo Giralt <jacobo.giralt@bsc.es>