ipv4: Do not use dead fib_info entries.

Due to RCU lookups and RCU based release, fib_info objects can
be found during lookup which have fi->fib_dead set.

We must ignore these entries, otherwise we risk dereferencing
the parts of the entry which are being torn down.
Reported-by: Yevgen Pronenko <yevgen.pronenko@sonymobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -1370,6 +1370,8 @@ static int check_leaf(struct fib_table *tb, struct trie *t, struct leaf *l,
 
 			if (fa->fa_tos && fa->fa_tos != flp->flowi4_tos)
 				continue;
+			if (fi->fib_dead)
+				continue;
 			if (fa->fa_info->fib_scope < flp->flowi4_scope)
 				continue;
 			fib_alias_accessed(fa);