pf: allocate krule->timestamp in pf_krule_alloc()

There are three calls which can allocate a new rule. DIOCADDRULE,
DIOCADDRULENV and DIOCCHANGERULE. The first two call pf_ioctl_addrule(),
but DIOCCHANGERULE does not. As a result rules created through
DIOCCHANGERULE do not have the timestamp per-cpu memory allocated, and
we panic when the rule is exported with pf_krule_to_nvrule().

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