pf: fix more syncookie memory leaks

Allocate memory for packed nvlists in M_NVLIST, as nvlist_pack() does
this as well, and we use the same variable interchangable with the
memory we allocate. When we free it we can end up freeing from the wrong
zone, leaking memory.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D35385