MFC r332107:

pf: Improve ioctl validation for DIOCRGETTABLES, DIOCRGETTSTATS, DIOCRCLRTSTATS and DIOCRSETTFLAGS

These ioctls can process a number of items at a time, which puts us at
risk of overflow in mallocarray() and of impossibly large allocations
even if we don't overflow.

Limit the allocation to required size (or the user allocation, if that's
smaller). That does mean we need to do the allocation with the rules
lock held (so the number doesn't change while we're doing this), so it
can't M_WAITOK.