ipfw(8): Handle unaligned pointers in pr_u64.

ipfw(8): Handle unaligned pointers in pr_u64.

struct _ipfw_dyn_rule is defined as packed, and as a result, its
uint64_t fields are misaligned on some 32-bit platforms.  Since
pr_u64() is explicitly supposed to handle this case, avoid using a
uint64_t * for the input pointer to make sure that the compiler won't
(correctly) warn about the misalignment.

Reported by:	jenkins
MFC with:	r363164