pf: Merge pf_clear_srcnodes() and pf_kill_srcnodes()

The functions pf_clear_srcnodes() and pf_kill_srcnodes() serve the same
purpose, however the former kills all source nodes while the later only
a selected subset of them.

They differ in how they reach that goal. pf_clear_srcnodes() first
iterates over all states and detaches the source nodes from them. Then
it iterates over all source nodes and marks them as expired leaving the
cleanup to pf_purge_expired_src_nodes().

If a new state and a new source node are created between iterating over
all states and all source nodes, this source node will have its state
counter set to 0 and expiry to 1, marking it as expired without properly
detaching the state from it. Later the source node will be freed with
the state sill pointing to it.

The function pf_kill_srcnodes() performs the same operation in a safer
manner by first marking the required source nodes as expiring and then
iterating over all states and checking which states point to expiring
nodes. Any source node created between iterating over states and source
nodes will simply be ignored.

Add functionality of killing all source nodes to pf_kill_srcnodes().
Replace all calls to pf_clear_srcnodes() with a calls to
pf_kill_srcnodes(), and remove the former.

Reviewed by: kp
Approved by: kp (mentor)
Differential Revision: https://reviews.freebsd.org/D47440