pf: pool/kpool conversion code

stuct pf_pool and struct pf_kpool are different. We should not simply
bcopy() them.

Happily it turns out that their differences were all pointers, and the
userspace provided pointers were overwritten by the kernel, so this did
actually work correctly, but we should fix it anyway.

Reviewed by: glebius
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29216

(cherry picked from commit 15b82e00a1640d1b9a1d720c95f65e580be30187)