[$] Concurrent page-fault handling with per-VMA locks

The kernel is, in many ways, a marvel of scalability, but there is alongstanding pain point in the memory-management subsystem that hasresisted all attempts at elimination: the mmap_lock. This lockwas inevitably a topic at the 2022 LinuxStorage, Filesystem, Memory-Management and BPF Summit (LSFMM), where the idea ofusing per-VMA locks was raised. Suren Baghdasaryan has postedan implementation of that idea - but with an interesting twist on howthose locks are implemented.