uma_prealloc() simply allocates slabs ahead of time. However, unless
uma_reserve() or uma_reserve_kva() is also called, or UMA_ZONE_NOFREE is
set, the uma_prealloc() call doesn't really accomplish anything since any
free slabs will be reclaimed well before slab allocations start to fail.
In the vm_map case, we specify UMA_ZONE_NOFREE, but the preallocation
does nothing to ensure that allocations will be successful. The
preallocation size is also quite small on modern systems. In the
siginfo case I think the preallocation accomplishes nothing. I propose
removing them.