Backout CSRG revision 7.22 to this file (if in_losing notices an

RTF_DYNAMIC route, it got freed twice).  I am not sure what was
the actual problem in 1992, but the current behavior is memory
leak if PCB holds a reference to a dynamically created/modified
routing table entry.  (rt_refcnt>0 and we don't call rtfree().)

My test bed was:

1.  Set net.inet.tcp.msl to a low value (for test purposes), e.g.,
    5 seconds, to speed up the transition of TCP connection to a
    "closed" state.
2.  Add a network route which causes ICMP redirect from the gateway.
3.  ping(8) host H that matches this route; this creates RTF_DYNAMIC
    RTF_HOST route to H.  (I was forced to use ICMP to cause gateway
    to generate ICMP host redirect, because gateway in question is a
    4.2-STABLE system vulnerable to a problem that was fixed later in
    ip_icmp.c,v 1.39.2.6, and TCP packets with DF bit set were
    triggering this bug.)
4.  telnet(1) to H
5.  Block access to H with ipfw(8)
6.  Send something in telnet(1) session; this causes EPERM, followed
    by an in_losing() call in a few seconds.
7.  Delete ipfw(8) rule blocking access to H, and wait for TCP
    connection moving to a CLOSED state; PCB is freed.
8.  Delete host route to H.
9.  Watch with netstat(1) that `rttrash' increased.
10. Repeat steps 3-9, and watch `rttrash' increases.

PR:		kern/25421
MFC after:	2 weeks

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