[$] How to get rid of mmap_sem

The mmap_sem lock used in the memory-management subsystem has been
a known scalability problem for years, but it has proved difficult to
remove. During a session in the memory-management track of the 2019 Linux
Storage, Filesystem, and Memory-Management Summit, Laurent Dufour and
Matthew Wilcox discussed a possible solution: replacing the red-black tree
currently used to track virtual memory areas (VMAs) with a new data
structure called a 'maple tree'.