fib: introduce fib_alias_accessed() helper

Perf tools session at NFWS 2010 pointed out a false sharing on struct
fib_alias that can be avoided pretty easily, if we set FA_S_ACCESSED bit
only if needed (ie : not already set)

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

diff --cc net/ipv4/fib_hash.c
Simple merge

diff --cc net/ipv4/fib_lookup.h
Simple merge

diff --cc net/ipv4/fib_semantics.c
Simple merge

diff --cc net/ipv4/fib_trie.c
Simple merge