Use call_rcu_bh() in route cache.  This allows faster grace periods and avoids
dst cache overflows during DoS testing.

This patch uses the call_rcu_bh() api in route cache code to facilitate
quicker RCU grace periods.  Quicker grace periods avoid overflow of dst cache
in heavily loaded routers as seen in Robert Olsson's testing.  
Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>