Switch AF_ROUTE sockets to the new locking scheme.

Switch AF_ROUTE sockets to the new locking scheme.

At sockets layer only mark buffers as SB_MTXLOCK. At PCB layer only
protect `so_rcv' with corresponding `sb_mtx' mutex(9).

SS_ISCONNECTED and SS_CANTRCVMORE bits are redundant for AF_ROUTE
sockets. Since SS_CANTRCVMORE modifications performed with both solock()
and `sb_mtx' held, the 'unlocked' SS_CANTRCVMORE check in
rtm_senddesync() is safe.

ok bluhm