IPv6: fix Mobile IPv6 regression

Commit f4f914b5 (net: ipv6 bind to device issue) caused
a regression with Mobile IPv6 when it changed the meaning
of fl->oif to become a strict requirement of the route
lookup.  Instead, only force strict mode when
sk->sk_bound_dev_if is set on the calling socket, getting
the intended behavior and fixing the regression.

Tested-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>