ipv4: Do not use dead fib_info entries.

ipv4: Do not use dead fib_info entries.

[ Upstream commit dccd9ecc374462e5d6a5b8f8110415a86c2213d8 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>