smalloc: limit to 1 pool, and bump size to 16MB

smalloc: limit to 1 pool, and bump size to 16MB

We can easily add new pools, but if we do this after jobs have been
forked off, then it wont really be shared storage as the new mmap
mappings will only apply to the process that instantiated them.
Since we moved the randommap mappings to private storage, we should
not need a lot of room for shared allocations. Hence I hope the 16MB
will suffice.

Signed-off-by: Jens Axboe <axboe@fb.com>