net: updating dst lastusage is an unlikely event.

Since commit 0da4af00

 ("ipv6: only update __use and lastusetime
once per jiffy at most"), updating the dst lastuse field is an
unlikely action: it happens at most once per jiffy, out of
potentially millions of calls per second.

Mark explicitly the code as such, and let the compiler generate
better code.

Note: gcc 7.2 and several older versions do actually generate
different - better - code when the unlikely() hint is in place,
avoid jump in the fast path and keeping better code locality.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>