net: sock: fix in-kernel mark setting

This patch fixes the in-kernel mark setting by doing an additional
sk_dst_reset() which was introduced by commit 50254256 ("sock: Reset
dst when changing sk_mark via setsockopt"). The code is now shared to
avoid any further suprises when changing the socket mark value.

Fixes: 84d1c617 ("net: sock: add sock_set_mark")
Reported-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>