net: Remove redundant oif checks in rt6_device_match

The oif has already been checked that it is non-zero; the 2 additional
checks on oif within that if (oif) {...} block are redundant.

CC: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>