suppress duplicated rtmsg on SMP system

[Sepherosa Ziehau]

Hi all,

Following patch suppress duplicated rtmsgs on SMP system:
http://leaf.dragonflybsd.org/~sephe/dup_rtmsg.diff

gitweb:
http://gitweb.dragonflybsd.org/~sephe/dragonfly.git/shortlog/refs/heads/route

Rest of the duplicated are produced during upper layer protocol output
process (PRCLONING and CLONING).  These duplicated rtmsgs are not
suppressed currently.

Best Regards,
sephe

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