net: speedup dst_release()

During tbench/oprofile sessions, I found that dst_release() was in third position.

CPU: Core 2, speed 2999.68 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit mask of 0x00 (Unhalted core cycles) count 100000
samples  %        symbol name
483726    9.0185  __copy_user_zeroing_intel
191466    3.5697  __copy_user_intel
185475    3.4580  dst_release
175114    3.2648  ip_queue_xmit
153447    2.8608  tcp_sendmsg
108775    2.0280  tcp_recvmsg
102659    1.9140  sysenter_past_esp
101450    1.8914  tcp_current_mss
95067     1.7724  __copy_from_user_ll
86531     1.6133  tcp_transmit_skb

Of course, all CPUS fight on the dst_entry associated with 127.0.0.1 

Instead of first checking the refcount value, then decrement it,
we use atomic_dec_return() to help CPU to make the right memory transaction
(ie getting the cache line in exclusive mode)

dst_release() is now at the fifth position, and tbench a litle bit faster ;)

CPU: Core 2, speed 3000.1 MHz (estimated)
Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit mask of 0x00 (Unhalted core cycles) count 100000
samples  %        symbol name
647107    8.8072  __copy_user_zeroing_intel
258840    3.5229  ip_queue_xmit
258302    3.5155  __copy_user_intel
209629    2.8531  tcp_sendmsg
165632    2.2543  dst_release
149232    2.0311  tcp_current_mss
147821    2.0119  tcp_recvmsg
137893    1.8767  sysenter_past_esp
127473    1.7349  __copy_from_user_ll
121308    1.6510  ip_finish_output
118510    1.6129  tcp_transmit_skb
109295    1.4875  tcp_v4_rcv

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>