pf: fix potential state key leak

If we fail to attach the new state keys in pf_state_key_attach() we are
expected to free both keys. Do so.
Note that in some scenarios both key are the same, so check for that
before freeing.

MFC after:	2 weeks
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D47495

sys/netpfil/pf/pf.c

@@ -1433,7 +1433,9 @@ keyattach:
 					s->timeout = PFTM_UNLINKED;
 					PF_HASHROW_UNLOCK(ih);
 					KEYS_UNLOCK();
-					uma_zfree(V_pf_state_key_z, sk);
+					uma_zfree(V_pf_state_key_z, skw);
+					if (skw != sks)
+						uma_zfree(V_pf_state_key_z, sks);
 					if (idx == PF_SK_STACK)
 						pf_detach_state(s);
 					return (EEXIST); /* collision! */