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.

Fixes: 9c125336727b ("pf: convert DIOCGETSRCNODES to netlink")
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")