pf: limit ioctl to a reasonable and tuneable number of elements

pf ioctls frequently take a variable number of elements as argument. This can
potentially allow users to request very large allocations. These will fail,
but even a failing M_NOWAIT might tie up resources and result in concurrent
M_WAITOK allocations entering vm_wait and inducing reclamation of caches.

Limit these ioctls to what should be a reasonable value, but allow users to
tune it should they need to.

Differential Revision: https://reviews.freebsd.org/D15018