Ktracing kevent(2) calls with unusual arguments might leads to an
overly large allocation requests.

When ktrace-ing io, sys_kevent() allocates memory to copy the
requested changes and reported events. Allocations are sized by the
incoming syscall lengths arguments, which are user-controlled, and
might cause overflow in calculations or too large allocations.

Since io trace chunks are limited by ktr_geniosize, there is no sense
it even trying to satisfy unbounded allocations. Export ktr_geniosize
and clamp the buffers sizes in advance.

PR: 217435
Reported by: Tim Newsham <tim.newsham@nccgroup.trust>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week