pf: do not leak a source hash row lock in the netlink dump

pf_handle_get_srcnodes() returns with the lock of a source hash row
held when it cannot start the message for a source node.  Unlock the
row there, as the other error exit of the loop does.

Reviewed by:		kp
Approved by:		kp (mentor)
Fixes:			9c12533672 ("pf: convert DIOCGETSRCNODES to netlink")
MFC after:		1 week
Sponsored by:		Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D60251

sys/netpfil/pf/pf_nl.c

@@ -1824,6 +1824,7 @@ pf_handle_get_srcnodes(struct nlmsghdr *hdr, struct nl_pstate *npt)
 
 		LIST_FOREACH(n, &sh->nodes, entry) {
 			if (!nlmsg_reply(nw, hdr, sizeof(struct genlmsghdr))) {
+				PF_HASHROW_UNLOCK(sh);
 				nlmsg_abort(nw);
 				return (ENOMEM);
 			}