[$] Slab allocator: sheaves and any-context allocations

The kernel's slab allocator is charged with providing small objects on
demand; its performance and reliability are crucial for the functioning of
the system as a whole. At the 2025 Linux Storage, Filesystem,
Memory-Management, and BPF Summit, two adjacent sessions in the
memory-management track dug into current work on the slab allocator. The
first focused on the new sheaves feature, while the second discussed a set
of allocation functions that are safe to call in any context.