sock: Reset dst when changing sk_mark via setsockopt

sock: Reset dst when changing sk_mark via setsockopt

When setting the SO_MARK socket option, if the mark changes, the dst
needs to be reset so that a new route lookup is performed.

This fixes the case where an application wants to change routing by
setting a new sk_mark.  If this is done after some packets have already
been sent, the dst is cached and has no effect.

Signed-off-by: David Barmann <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Signed-off-by: David S. Miller <[email protected]>