ipfilter: Don't trust userland supplied iph_size

ipf_htable_create() trusts a user-supplied iph_size from iphtable_t
and computes the allocation size as iph->iph_size * sizeof(*iph->iph_table)
without checking for integer overflow. A sufficiently large iph_size
causes the multiplication to wrap, resulting in an under-sized allocation
for the table pointer array. Subsequent code (e.g., in ipf_htent_insert())
can then write past the end of the allocated buffer, corrupting kernel
memory and causing DoS or potential privilege escalation.

This is not typically a problem when using the ipfilter provided
userland tools as calculate the correct lengths. This mitigates a
rogue actor calling ipfilter ioctls directly.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D53286

(cherry picked from commit df381bec2d2b73697a3d163177df042dd272022d)