fib: remove a useless synchronize_rcu() call

fib_nl_delrule() calls synchronize_rcu() for no apparent reason,
while rtnl is held.

I suspect it was done to avoid an atomic_inc_not_zero() in
fib_rules_lookup(), which commit 7fa7cb71 added anyway.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>