fib: Change data type of fib oifname to "ifname"

fib: Change data type of fib oifname to "ifname"

Change data type of fib oifname from "string" to "ifname", so that it
can be matched against a set of ifnames:

    set x {
            type ifname
    }
    chain y {
            fib saddr oifname @x drop
    }

Signed-off-by: Xiao Liang <shaw.leon@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>