[NET]: dst_ifdown() cleanup

This cleanup shrinks size of net/core/dst.o on i386 from 1299 to 1289 bytes.
(This is because dev_hold()/dev_put() are doing atomic_inc()/atomic_dec() and
force compiler to re-evaluate memory contents.)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>