pf: fix memory leak

The nvlist is allocated in pf_keth_rule_to_nveth_rule(). There's no need
to allocate one in the calling function. Especially not as we overwrite
the pointer to the new nvlist with the one allocated by
pf_keth_rule_to_nveth_rule(), leaking memory.

Reported by: Coverity
CID: 1476128
Sponsored by: Rubicon Communications, LLC ("Netgate")