[$] Improvements for the contiguous memory allocator

As a system runs, its memory becomes fragmented; it does not take longbefore the allocation of large, physically contiguous memory ranges becomesdifficult or impossible. The contiguous memoryallocator (CMA) is a kernel subsystem that attempts to address thisproblem, but it has never worked as well as some would like. Two sessionsin the memory-management track at the 2025 Linux Storage, Filesystem,Memory-Management, and BPF Summit looked at how CMA can be improved; thefirst looked at providing guaranteed allocations, while the secondaddressed some inefficiencies in CMA.