pipex(4) prevent `old_session_keys' memory leak

[Vitaliy Makkoveev]

Before session freeing pipex_rele_session() will check and release
`old_session_keys' if necessary. So use it instead of pool_put(9) within
pipex_destroy_session().

Index: sys/net/pipex.c
===================================================================
RCS file: /cvs/src/sys/net/pipex.c,v
retrieving revision 1.117
diff -u -p -r1.117 pipex.c
--- sys/net/pipex.c     30 Jun 2020 14:05:13 -0000      1.117
+++ sys/net/pipex.c     6 Jul 2020 13:23:25 -0000
@@ -652,7 +652,7 @@ pipex_destroy_session(struct pipex_sessi
        }
 
        pipex_unlink_session(session);
-       pool_put(&pipex_session_pool, session);
+       pipex_rele_session(session);
 
        return (0);
 }