dm snapshot: improve performance by switching out_of_order_list to rbtree

copy_complete()'s processing of out_of_order_list can result in
quadratic complexity in the worst case.  As such it was the source of
consuming too much cpu and the source of significant loss in
performance.

Fix this by converting out_of_order_list to an rbtree.  This improved
a dm-snapshot test copy workload from 32 seconds to 4 seconds.

Signed-off-by: David Jeffery <djeffery@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Tested-by: Brett Hull <bhull@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>